用 HttpWorkerRequest ！
用它的GetPreloadedEntityBody 和 ReadEntityBody方法，它会从IIS为.NET建立pipe并分块读取数据
IServiceProvider provider = (IServiceProvider) HttpContext.Current;
HttpWorkerRequest wr = (HttpWorkerRequest) provider.GetService(typeof(HttpWorkerRequest));
  byte[] bs = wr.GetPreloadedEntityBody();
  ....
  if (!wr.IsEntireEntityBodyIsPreloaded())
  {
        int n = 1024;
        byte[] bs2 = new byte[n];
        while (wr.ReadEntityBody(bs2,n) >0)
       {
             .....
        }
  }
<引用>

也可以改配置文件
<httpRuntime  
executionTimeout="400"  
maxRequestLength="40960"  
useFullyQualifiedRedirectUrl="false"/>