<TextBox>
    <TextBox.Resources>
        <VisualBrush x:Key="HelpBrush" TileMode="None" Opacity="0.3" Stretch="None" AlignmentX="Left">
            <VisualBrush.Visual>
                <TextBlock FontStyle="Italic" Text="请输入用户名"/>
            </VisualBrush.Visual>
        </VisualBrush>
    </TextBox.Resources>
    <TextBox.Style>
        <Style TargetType="TextBox">
            <Style.Triggers>
                <Trigger Property="Text" Value="{x:Null}">
                    <Setter Property="Background" Value="{StaticResource HelpBrush}"/>
                </Trigger>
                <Trigger Property="Text" Value="">
                    <Setter Property="Background" Value="{StaticResource HelpBrush}"/>
                </Trigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
</TextBox>

参考

WPF TextBox Placeholder - Komp - OSCHINA - 中文开源技术交流社区