Abstract: It is suggested in this that: there is a generation of vigorous hydromagnetic waves when the magnetic field is disturbed by the fluid field of the sunspot.In the processes,the thermal energy converted into wave energy of hydromagnetic waves which rapidly propagate out of the sunspot region with Alfven speed along the magnetic field,hence resulting in cooling of the sunspot.It is very effective to keep observed temperature of the sunspot umbra at surface of the photosphere when the component of disturbed velosity normal the magnetic field is only 0.4 km/s.