Container(
  decoration: BoxDecoration(
                  shape: BoxShape.circle,
                  gradient: LinearGradient(
                  colors: [
                          color1,
                          color2,
                  ],
                  begin: Alignment.bottomLeft,
                  end: Alignment.topRight
                  )
              ),
              child: CircleAvatar(
                  child: Text(
                        'MS',
                        style: TextStyle(
                          color: Colors.white
                        )
                  ),
                  backgroundColor: Colors.transparent
              )
    )

通过将Container作为CircleAvatar的子项，并将gradient属性赋予Container，我实现了我想要的功能。

CircleAvatar(
        radius: 40,
        child: Container(
          decoration: BoxDecoration(
            shape: BoxShape.circle,
            gradient: LinearGradient(
              colors: [
                color1,
                color2,
              ],
            ),
          ),
        ),
      ),

首先，我用Container Package CircleAvatar，并给予形状和渐变色赋予Container，将透明色赋予CircleAvatar。

child: Container(
  decoration: BoxDecoration(
  gradient: LinearGradient(
    colors: [Colors.red, Colors.orange]),
    shape: BoxShape.circle,
  ),
  child: CircleAvatar(
    backgroundColor: Colors.transparent,
    radius: 45,
  ),
),