An all-optical light modulation scheme based on improved prism-waveguide coupling method is developed. All-optical modulation is demonstrated due to the thermo-optic effect of anthraquinone organic dye-doped polymethyl-methacrylate films. The pump light is incident upon the top of the prism perpendicularly,so that the probe and pump lights are easy to be coupled into the waveguide films. The intensity of the probe light is modulated by the intensity of the pump light and the response time is about 20ms. The scheme can also be applied to all-optical modulation or switch based on the third-order nonlinear Kerr effect which has shorter response time.