private void Form1_Load(object sender, EventArgs e)
        {
            SqlConnection sql = new SqlConnection(@"Data Source=LIUFENG\SQLEXPRESS;Initial Catalog=liufeng;Integrated Security=True");
            SqlCommand cmd = new SqlCommand("Select * from TreeTable", sql);
            DataSet dss = new DataSet();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.Fill(dss);
            DataTable dt = dss.Tables[0];
            BindTree(gvTreeView, dt);
        }
        protected void BindTree(gvTreeView gvTreeView , DataTable dt)
        {
            gvTreeView.Nodes.Clear();//清空树节点
            DataRow []row = dt.Select("ParentID is null");
            if (row.Length <= 0)
            {
                return;//找不到根结点则返回
            }
            gvTreeNode root = new gvTreeNode();
            root.Text = row[0]["name"].ToString();
            root.Tag = row[0]["ID"].ToString();            
            gvTreeView.Nodes.Add(root);           
            CreateChildNode(root, dt);
            root.Expand();
        }

        protected void CreateChildNode(gvTreeNode gvTreeNode, DataTable dt)
        {
            DataRow[] rows = dt.Select("ParentID ='" + gvTreeNode.Tag + "'");
            foreach(DataRow row in rows)
            {
                gvTreeNode Node = new gvTreeNode();
                Node.Text = row["name"].ToString();
                Node.Tag = row["ID"].ToString();
                gvTreeNode.Nodes.Add(Node);
                CreateChildNode(Node, dt);
            }
        }

------解决方案--------------------
treeView1.ExpandAll()
就可以展开啊。