The traditional method of optical difference frequency THz radiation with low conversion efficiency can not get high power THz output.In this paper,DFG THz radiation is calculated in quasi-phase-matched（QPM）GaAs.Tunable THz wave is obtained from QPM GaAs crystals using the temperature tuning method.In order to improve the gain of DFG and quantum efficiency,the mechanism of cascaded difference frequency is introduced,and the optimum crystal length and the pump frequency are calculated.It is obviously seen from the results that the terahertz conversion efficiency and the intensity can be greatly enhanced in QPM GaAs.