把 <system.web> 里的ajax 配置 转移到<system.webServer> 就OK 了

<system.web>

</system.web>

<system.webServer>

<modules runAllManagedModulesForAllRequests="true"/>
<security>
  <requestFiltering>
    <requestLimits maxAllowedContentLength="1048576000"></requestLimits>
  </requestFiltering>
</security>
<staticContent>
  <mimeMap fileExtension=".*" mimeType="text/xml" />
</staticContent>
<handlers>
  <add name="ajax"  verb="POST,GET" path="ajax/*.ashx" type="Ajax.PageHandlerFactory, Ajax" />
  <remove name="WebServiceHandlerFactory-Integrated" />
  <remove name="ScriptHandlerFactory" />
  <remove name="ScriptHandlerFactoryAppServices" />
  <remove name="ScriptResource" />
  <add name="ScriptHandlerFactory" verb="*" path="*.asmx" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
  <add name="ScriptHandlerFactoryAppServices" verb="*" path="*_AppService.axd" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
  <add name="ScriptResource" preCondition="integratedMode" verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
</handlers>

</system.webServer>