private string GetFileNameAndPost(FileUpload fld,Label msg)
    {
        string fName = "";
        if (fld.FileName.Trim() == "" || !(fld.FileName))
        {
            msg.Text = "未上传";
            return "";
        }
        else
        {
            string fname = fld.PostedFile.FileName;
            //if(fld.PostedFile.ContentType !=
            if (fname.LastIndexOf("\\") > -1)
            {
                fname = fname.Substring(fname.LastIndexOf("\\") + 1);
                fld.PostedFile.SaveAs(Server.MapPath("system\\cus_Picture\\" + fname));
                fName = fname;
                msg.Text = fName + "上传成功";
            }
            else
            {
                msg.Text = fname + "上传失败";
                return "";
            }
        }
        return fName;
    }










    protected void Button4_Click(object sender, EventArgs e)
    {
        GetFileNameAndPost(FileUpload1, Label1);
        GetFileNameAndPost(FileUpload2, Label2);
        GetFileNameAndPost(FileUpload3, Label3);
    }

!(fld.FileName))原来这句是检测服务器的文件。。
解决了,郁闷啊。