<script language="JavaScript" type="text/javascript">
function js(id1,id2,id4)
{
 var o1 = document.getElementById(id1);
 var o2 = document.getElementById(id2);
 var o4 = document.getElementById(id4);
  total = parseInt(o1.value)*parseInt(o2.value);
 o4.value =total;
}
</script>
<form name="form1" method="post" action="">
      <table width="96%"  border="0" align="center" cellpadding="4" cellspacing="1" bgcolor="#aec3de">
    <tr>
      <td>序号</td>
      <td>项目</td>
      <td>单价</td>
      <td>数量</td>
      <td>总金额</td>
      <td>备注</td>
    </tr>   
        <%
        sql="select * from zxc "
        set rs=server.createobject("adodb.recordset")
       rs.open sql,conn,1,1
       Do While Not rs.eof         
        %>

    <tr align="center" bgcolor='#F2FDFF'>
      <td width="5%" align='left'><%=rs("id")%></td>
      <td width="10%" align='left'><%=rs("xiangmu")%></td>
      <td width="10%" align='left'><input name="danjia" type="text" id="Ya" value=<%=rs("danjia")%> onChange="js('Ya','Sa','Ta')"></td>
      <td width="10%" align='left' ><input name="shuliang" type="text" id="Sa"  onChange="js('Ya','Sa','Ta')"></td>
      <td width="10%" align='left'><input name="zongjine" type="text" id="Ta" onChange="js('Ya','Sa','Ta')"></td>
      <td width="55%" align='left'><%=rs("beizhu")%></td>

    </tr>
    <%
        rs.movenext
        Loop
        rs.close
        Set rs=Nothing
    %>
  </table>

网站开发基础常识：网页中的id，不允许重复。
改为
<td width="10%" align='left'><input name="danjia" type="text" id="Ya_<%=rs("id")%>" value=<%=rs("danjia")%> onChange="js('Ya_<%=rs("id")%>','Sa_<%=rs("id")%>','Ta_<%=rs("id")%>')"></td>
<td width="10%" align='left' ><input name="shuliang" type="text" id="Sa_<%=rs("id")%>"  onChange="js('Ya_<%=rs("id")%>','Sa_<%=rs("id")%>','Ta_<%=rs("id")%>')"></td>
<td width="10%" align='left'><input name="zongjine" type="text" id="Ta_<%=rs("id")%>" onChange="js('Ya_<%=rs("id")%>','Sa_<%=rs("id")%>','Ta_<%=rs("id")%>')"></td>