1. 方法1
<Separator BorderBrush="Gray">
<Separator.LayoutTransform>
<RotateTransform Angle="90" />
</Separator.LayoutTransform>
</Separator>
2. 方法2
<Separator Style="{StaticResource {x:Static ToolBar.SeparatorStyleKey}}"
BorderBrush="Gray" />
参考
WPF如何制作一个竖直的separator