可以用javascript
<script language="javascript">
function var_BM_Change(val)
{
alert('选中的值为'+val);
}
</script>
<select
id="var_BM" onChange="var_BM_Change(this.options[this.selectedIndex].value);" >
<%
dim rsbm
set rsbm = server.CreateObject("adodb.recordset")
sql="select * from HR_BM"
rsbm.open sql,conn,1,1
while not rsbm.eof
%>
<option value="<%=rsbm("BM_ID")%>" <%if cint(rsbm("BM_ID"))=cint(rs("User_BM")) then response.write"selected" end if%>>
<%=rsbm("BM_Class")%></option>
<%
rsbm.movenext
wend
rsbm.close
set rsbm = nothing
%>
</select>