首先添加引用Microsoft.mshtml，具体操作项目->添加引用->.NET找到添加就行

需要的控件webbrowser一个，button一个，如果名字不是默认请修正代码中控件名字。

然后代码如下即可：在vs2010,【.net4.0下调试通过】
Private Sub Button1_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles Button1.Click
        '获取用户选中的文字
    Dim htmlDocument As mshtml.IHTMLDocument2  'IHTMLDocument2
        htmlDocument = WebBrowser1.Document.DomDocument
        Dim currentSelection As mshtml.IHTMLSelectionObject
        currentSelection = htmlDocument.selection
        Dim range As mshtml.IHTMLTxtRange

        If Not (currentSelection Is Nothing) Then
            range = currentSelection.createRange()
            If Not (range Is Nothing) Then
                MsgBox(range.text)       ‘可以进行你希望的操作       
            End If
        End If
    End Sub