I think I find another way to solve the problem. First, I add a new function in the ASP Header:
function setClientID(id) { this.clientID = id; }
Then, add the below code to code-behind after creating dropdownlist:
Page.ClientScript.RegisterStartupScript(this.GetType(), "setClientID", "setClientID('" + mySelect.ClientID + "');", true);
Finally, user "this.ClientID" to replace the "<%=mySelect.ClientID%>". It works.