why when I press a Ctrl+Tab in a multiline TextBox,it enters a tab character, even i set the AcceptsTab property to false?

Because the AcceptsTab property only affects the TAB key - CTRL+TAB is a different keystroke, which is always inserted into a TextBox. If you want to prevent it, then look at handling the Keydown event:

private void myTextBox_KeyDown(object sender, KeyEventArgs e)
    {
    if (e.KeyCode == Keys.Tab && e.Control)
        {
        e.Handled = true;
        }
    }