int i = 0;

        private void button1_Click_1(object sender, EventArgs e)
        {
            i = 0;
            BrowseDropdownlistItem();

        }

        private void BrowseDropdownlistItem()
        {
            if (i < comboBox1.Items.Count)
            {
                string url = comboBox1.Items[i].ToString();
                webBrowser1.Navigate(url);
            }
        }

        private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            //避免webbroswer.DocumentCompleted被多次引发
            if ((e.Url != webBrowser1.Url) || (webBrowser1.ReadyState != WebBrowserReadyState.Complete))
                return;
            //下面写你要在页面加载完毕后执行的代码。

            i++;
            BrowseDropdownlistItem();
        }