<script>
function dosubmit(){
   var f = document.forms[0];
   f.textarea.value = ccc.document.forms[0].textarea.value;
   f.submit();
}
</script>
<form action="Asevlet" ENCTYPE="multipart/form-data" METHOD="POST"> 
<input name="Title1" type="text" > 
<input name="Title2" type="text" > 
<input name="Title3" type="text" > 
<input name="textarea" type="hidden"> 
<iframe name="ccc" src="b.jsp"> </iframe> 
<input name="Submit" type="button" value="提交" onclick="dosubmit()"> 
</form>

------解决方案--------------------
windows.frames[0].document.getElementsByName("textarea").value;