step 1
replace the tag of with the follewing codes:

<%
  if (Request["__SCROLLPOS"] != null &&
      Request["__SCROLLPOS"] != String.Empty) {
      int pos = Convert.ToInt32 (Request["__SCROLLPOS"]);
      Response.Write ("          "onscroll=\"javascript:document.forms[0].__SCROLLPOS.value = " +
          "theBody.scrollTop;\" " +
          "onload=\"javascript:theBody.scrollTop=" + pos + ";\">");
  }
  else {
      Response.Write ("          "onscroll=\"javascript:document.forms[0].__SCROLLPOS.value =" +
          "theBody.scrollTop;\">");
  }
%>

step 2
add the follewing codes between and

或是
<@ page smartnavigation=true%>   比较好用，经常用