Abstract: Design and simulations of the CAEP high power THz free electron laser(FEL) have been carried out using our 3D-Osifel code. The main work focuses on the optimization of different schemes through physical analysis. The wiggler peak field strength and electron beam energy have been selected with eleven frequencies ranging from 1 THz to 3 THz. The gain and power of resonator are calculated, which are corresponding to eleven frequencies. It can be seen that when the facility works at 2.6 THz, the gain and output power are high. So the experiment may be conducted at this frequency first. When the device works at 1 THz, the slippage effect between the electron beam and radiation increases due to the increase of the radiation wavelength, which results in lack of overlapping between the electron and the optical pulses, then, the gain and output power lower. So experiment running at 1 THz could be challenged and further probe work should been put forward to solve the problem.