一、实现IHttpModule类，见后。

二、修改web.config
添加域名
<appSettings><add key="RootDomain" value=""/></appSettings>
RootDomain cookie域名，若是本地则写空
by design domain names must have at least two dots otherwise browser will say they are invalid 
when working on localhost (!) the cookie-domain must be set to "" or NULL or FALSE instead of "localhost"

增加过滤
<system.web>
<sessionState mode="StateServer" stateNetworkTimeout="10" timeout="120" stateConnectionString="tcpip=127.0.0.1:42424" />
 <httpModules>

      <add name="SessionSharedHttpModule" type="ClassLibrary.BasePage.MakeSessionIDOneOnly,ClassLibrary"/>

    </httpModules>
</system.web>

三、附类
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.SessionState;
using System.Reflection;
using System.Configuration;

namespace ClassLibrary.BasePage
{
    public class MakeSessionIDOneOnly : IHttpModule
    {
        private string m_RootDomain = string.Empty;

        #region IHttpModule Members

        public void Dispose()
        {

        }

        public void Init(HttpApplication context)
        {
            m_RootDomain = ConfigurationManager.AppSettings["RootDomain"];

            Type stateServerSessionProvider = typeof(HttpSessionState).Assembly.GetType("System.Web.SessionState.OutOfProcSessionStateStore");
            FieldInfo uriField = stateServerSessionProvider.GetField("s_uribase", BindingFlags.Static | BindingFlags.NonPublic);

            if (uriField == null)
                throw new ArgumentException("UriField was not found");

            uriField.SetValue(null, m_RootDomain);

            context.EndRequest += new System.EventHandler(context_EndRequest);
        }

        void context_EndRequest(object sender, System.EventArgs e)
        {
            HttpApplication app = sender as HttpApplication;
            for (int i = 0; i < app.Context.Response.Cookies.Count; i++)
            {
                if (app.Context.Response.Cookies[i].Name == "ASP.NET_SessionId")
                {
                    app.Context.Response.Cookies[i].Domain = m_RootDomain;
                }
            }
        }

        #endregion
    }
}