<TextBlock
Grid.Row="7" HorizontalAlignment="Center" VerticalAlignment="Center"
Style="{StaticResource LoginTitle}"
FontSize="21" Foreground="#3C6368"
Block.TextAlignment="Center"
TextWrapping="WrapWithOverflow">
aaaaa<LineBreak/>
bbbbb<LineBreak/>
</TextBlock>
设置Block.TextAlignment="Center"即可