protected void Page_Load(object sender, EventArgs e)
{
    if(IsPostBack)
    {
        Boolean fileOK = false;
        String path = Server.MapPath("~/UploadedImages/");
        if (FileUpload1.HasFile) 
        {
            String fileExtension = 
                System.IO.Path.GetExtension(FileUpload1.FileName).ToLower();
            String[] allowedExtensions = 
                {".gif", ".png", ".jpeg", ".jpg"};
          for (int i = 0; i < allowedExtensions.Length; i++)
          {
               if (fileExtension == allowedExtensions[i])
               {
                    fileOK = true;
               }
          }
        }

        if (fileOK)
        {
            try
            {
                FileUpload1.PostedFile.SaveAs(path 
                    + FileUpload1.FileName);
                Label1.Text = "File uploaded!";
            }
            catch (Exception ex)
            {
                Label1.Text = "File could not be uploaded.";
            }
        }
        else
        {
            Label1.Text = "Cannot accept files of this type.";
        }
    }
}

------解决方案--------------------
提示：把获取文件路径写在一个方法中，上传和读取文件的路径时调用这个方法就可以。。
 public static string GetUploadFilePath(Channels channel, string fileName, string extend, bool isUpload)
       {
        ............
         string folder = fileName.Substring(0, 1);

           string directory = string.Format("{0}Img\\{1}\\{2}", HttpContext.Current.Request.MapPath("/"), channelPath, folder);

           if (!Directory.Exists(directory))
               Directory.CreateDirectory(directory);

           if (isUpload)
               fullPath = string.Format("{0}\\{1}{2}", directory, fileName, extend);
           else
               fullPath = string.Format("/Img/{0}/{1}/{2}{3}", channelPath, folder, fileName, fullExtend);

           return fullPath;
        }
在其它地方调用这个方法即可。。。
string path=GetUploadFilePath(Channels channel, string fileName, string extend, bool isUpload);