The metal spherical shell constitutes an equipotential surface (because there are free electrons on the metal, if there is an electric field (or electric field force) in the tangential direction, then the electrons will move, so the electric field on the metal surface must be perpendicular to the metal surface in the equilibrium state). After the spherical shell is grounded, the potential of the spherical shell is 0. Assuming that there is a field point inside the spherical shell that is not zero, the Gaussian integral is not zero, which violates Gaussian law.