after about ten hours ,I slove the problem.

the solution is :

in mojoSiteMapProvider.cs        add some code like this :

  public override SiteMapNode CurrentNode
        {
            get
            {

                PageSettings a = CacheHelper.GetCurrentPage();

              
                SiteMapNode node = CreateSiteMapNode(a, 0);

                return node;
            }
        }

Thansk joe.    mojoporta is very excellent.    Sorry for my english  :)

HI Joe .Thanks for your reply.

You are right.

but if I don't modify mojoSiteMapProvider.cs  ,I can't solve the problem.

I  only  used the sitemenu control  in layout.master.

I don't konw where to add the SiteUtils.GetCurrentPageSiteMapNode() code.

May be I should create another  SiteMapProvider or Sitemenu?     Thanks

Hi  joy ,Thanks for your reply.

the article describe my scenario:

Visually Partitioning Sites So That Sections Appear To Be Separate Sites
 

I want to divide a site into sections that appear to be independent sites but have a common user base. 
 The example scenario below gives an idea of how to configure mojoPortal to meet these needs.

 University Site
--School of Foo (Section Home)
--School of Bar (Section home)
--School of Music (Section home)

 with each section having its own skin   In fact each section home has a split menu skins with a top horizntal menu showing sub sections within the section

 Basic steps:

 1. 2.3.....

 4. Set the StartingNodeOffset=1 on the top SiteMenu, this makes the
 top menu bind to node

 <portal:SiteMenu id="SiteMenu1" runat="server"
 UseTreeView="false"
 TopLevelOnly="true"
 StartingNodeOffset="1"
 Direction="Horizontal">
 </portal:SiteMenu>

in   section home,   I add  a module like  blog.  when I view the blog article ,in default. the sitemenu doesn't show.

Thanks