AboutBox dialog = new AboutBox();
dialog.Top = mainWindow.Top + 20;

要让它居中，您也可以使用WindowStartupLocation属性。设置为WindowStartupLocation.CenterOwner

AboutBox dialog = new AboutBox();
dialog.Owner = Application.Current.MainWindow; // We must also set the owner for this to work.
dialog.WindowStartupLocation = WindowStartupLocation.CenterOwner;

如果您希望将其水平居中，但不是垂直居中(即固定的垂直位置)，则在加载关于框之后，您必须在EventHandler中执行此操作，因为您需要根据AboutBox的宽度来计算水平位置，这仅在加载后才知道。

protected override void OnInitialized(...)
{
    this.Left = this.Owner.Left + (this.Owner.Width - this.ActualWidth) / 2;
    this.Top = this.Owner.Top + 20;
}

gehho。