对command1_click代码修改如下可达到你的要求:
Private Sub Command1_Click() '打开表
  Dim Fname As String
  CommonDialog1.ShowOpen
  Fname = CommonDialog1.FileName
  Text1.Text = Fname
  If Not (XlApp Is Nothing) Then
    xlBook.Close
    XlApp.Quit
  End If
  Set XlApp = CreateObject("Excel.Application") '创建EXCEL应用类
  XlApp.Visible = True '设置EXCEL可见
  Set xlBook = XlApp.Workbooks.Open(Text1.Text) '打开EXCEL工作簿
  Combo1.Clear
  For Each xlsheet In xlBook.Worksheets
    Combo1.AddItem xlsheet.Name
  Next xlsheet
  'For i = 0 To Combo1.ListCount - 1
    Combo1.Text = Combo1.List(0)
  'Next
  Set xlsheet = xlBook.Worksheets(1) '打开EXCEL工作表
  xlsheet.Activate '激活工作表
  Me.SetFocus
End Sub