在页面FormMain_Load里面添加如下注册事件：

            for (int i = 0; i < this.toolStrip1.Items.Count; i++) {
                this.toolStrip1.Items[i].MouseHover += new EventHandler(toolStrip1_MouseHover);
            }

添加方法：

       private void toolStrip1_MouseHover(object sender, EventArgs e) {
            if (sender is ToolStripDropDownItem) {
                ToolStripDropDownItem item = sender as ToolStripDropDownItem;
                if (item.HasDropDownItems && !item.DropDown.Visible) {
                    item.ShowDropDown();
                }
            }

        }