需在 Card 组件中显式申明：clipBehavior: Clip.antiAlias,，完整代码如下：

class _CardItemState extends State {
  void _clicked() {}
  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.all(10),
      // height: 200,
      child: new Card(
        clipBehavior: Clip.antiAlias,
        child: Column(
          children: [
            Image.asset(
              "images/userBgImag.png",
              // fit: BoxFit.cover,
            ),
          ],
        ),
      ),
    );
  }
}