您可以尝试这种方式:

floatingActionButton: FloatingActionButton(
      onPressed: () {},
      backgroundColor: Colors.red,
      elevation: 0,
      child: Container(
        decoration: BoxDecoration(
          color: Colors.transparent,
          borderRadius: BorderRadius.all(
            Radius.circular(100),
          ),
          boxShadow: [
            BoxShadow(
              color: Colors.purple.withOpacity(0.3),
              spreadRadius: 7,
              blurRadius: 7,
              offset: Offset(3, 5),
            ),
          ],
        ),
      ),
    ),