方式一：代码
ServicePointManager.DefaultConnectionLimit = 10
方式二：配置
<system.net> <connectionManagement> <add address = "http://www.contoso.com" maxconnection = "4" /> <add address = "*" maxconnection = "2" /> </connectionManagement> </system.net>
原文：http://www.cnblogs.com/zanxiaofeng/p/3934848.html