how a mvc web project backend access to another mvc api project

there are tow asp.net mvc projects.one is mvc api and another is mvc web. the mvc api using owin for auth2. and the mvc web access the api backed,not js. the problem is ,when web (backed) accesses the api ,there will be no response,but if i use a desktop win form client ,it works. i want to know why,and how to fix it.

some code ,and packages ,vs2015

owin client side

private HttpClient _client;
....
    private async Task<TokenParam> GetAccessToken(string user,string psw)
    {
        var parameters = new Dictionary<string,string>();
        parameters.Add("grant_type","password");
        parameters.Add("username",user);
        parameters.Add("password",psw);

        _client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", Convert.ToBase64String(Encoding.ASCII.GetBytes(Id + ":" + Secret)));

        var response = await _client.PostAsync("/token",new FormUrlEncodedContent(parameters));
        var value = await response.Content.ReadAsStringAsync();-----> here no response back from api server

        if (response.StatusCode == HttpStatusCode.OK)
        {
            var o = value.FromJson<TokenParam>();
            o.LoginTime = DateTime.Now.AddSeconds(-10);
            return o;
        }
        return null;
    }

owin server side

public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
    {
        var oAuthIdentity = new ClaimsIdentity(context.Options.AuthenticationType);
        oAuthIdentity.AddClaim(new Claim(ClaimTypes.Name,context.UserName));
        var ticket = new AuthenticationTicket(oAuthIdentity,new AuthenticationProperties());
        context.Validated(ticket);
        await base.GrantResourceOwnerCredentials(context);
    }

my packages.config file

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="Antlr" version="3.4.1.9004" targetFramework="net452" />
  <package id="bootstrap" version="3.0.0" targetFramework="net452" />
  <package id="EntityFramework" version="6.1.3" targetFramework="net452" />
  <package id="jQuery" version="1.10.2" targetFramework="net452" />
  <package id="jQuery.Validation" version="1.11.1" targetFramework="net452" />
  <package id="Microsoft.AspNet.Identity.Core" version="2.2.1" targetFramework="net452" />
  <package id="Microsoft.AspNet.Identity.EntityFramework" version="2.2.1" targetFramework="net452" />
  <package id="Microsoft.AspNet.Identity.Owin" version="2.2.1" targetFramework="net452" />
  <package id="Microsoft.AspNet.Mvc" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.Razor" version="3.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.Web.Optimization" version="1.1.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi.Client" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi.Core" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi.HelpPage" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi.Owin" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi.WebHost" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebPages" version="3.2.3" targetFramework="net452" />
  <package id="Microsoft.CodeDom.Providers.DotNetCompilerPlatform" version="1.0.0" targetFramework="net452" />
  <package id="Microsoft.jQuery.Unobtrusive.Validation" version="3.2.3" targetFramework="net452" />
  <package id="Microsoft.Net.Compilers" version="1.0.0" targetFramework="net452" developmentDependency="true" />
  <package id="Microsoft.Owin" version="3.0.1" targetFramework="net452" />
  <package id="Microsoft.Owin.Host.SystemWeb" version="3.0.1" targetFramework="net452" />
  <package id="Microsoft.Owin.Security" version="3.0.1" targetFramework="net452" />
  <package id="Microsoft.Owin.Security.Cookies" version="3.0.1" targetFramework="net452" />
  <package id="Microsoft.Owin.Security.Facebook" version="3.0.1" targetFramework="net452" />
  <package id="Microsoft.Owin.Security.Google" version="3.0.1" targetFramework="net452" />
  <package id="Microsoft.Owin.Security.MicrosoftAccount" version="3.0.1" targetFramework="net452" />
  <package id="Microsoft.Owin.Security.OAuth" version="3.0.1" targetFramework="net452" />
  <package id="Microsoft.Owin.Security.Twitter" version="3.0.1" targetFramework="net452" />
  <package id="Microsoft.Web.Infrastructure" version="1.0.0.0" targetFramework="net452" />
  <package id="Modernizr" version="2.6.2" targetFramework="net452" />
  <package id="Newtonsoft.Json" version="6.0.4" targetFramework="net452" />
  <package id="Owin" version="1.0" targetFramework="net452" />
  <package id="Respond" version="1.2.0" targetFramework="net452" />
  <package id="WebGrease" version="1.5.2" targetFramework="net452" />
</packages>