In 2002,a reservoir induced earthquake sequence took place in Shanxi reservoir,Wenzhou city,Zhejiang province.The corner frequencies of the earthquakes were prominently less than that of tectonic earthquakes.Based on the theory of wave propagation in two phases medium,the variations of wave velocity, v_p/v_s ratio and the influence on corner frequency in the process of dilatancy-diffusion, saturation with water and strength weakness of rock mass are discussed.According to some experiment results if the porosity is 0.15 and the elasticity modulus of rock decreases 23%, the corner frequency can decrease about 29%.It indicates that the corner frequency of the reservoir-induced earthquake decreasing is caused by fracture growing and infiltration diffusion of water,which lead to the strength weakness of rock mass.Thus the corner frequence may be used as a criterion for distinguishing the reservoir-reduced earthquakes from the tectonic earthquakes.