Numerical studies of terahertz （THz） radiation based on difference frequency generation in GaSe crystal are reported. When the gain saturation is achieved at a wavelength of 227.5 μm，which corresponds to a crystal length of 26.3 mm，the peak output power for THz radiation reaches 945 W. As a result of crystal absorption，the gain saturation region of THz radiation is the unstable output region，which is in front of the stable output region; and the stability of THz radiation is determined by the stability of pump. The peak output power for THz radiation reaches 735 W at the wavelength of 227.5 μm and at the corresponding crystal length of 37.9 mm in stable region.