Designing stable and reliable THz lasers as well as reducing dimension of THz lasers have always been a hot research spot in THz field. Based on the rate equations, the model of optically pumped THz lasers has been built up. The effects of the operating temperature, the pressure in cavity, the dimension of cavity and the stability of pump laser on the output characteristics of optically pumped THz lasers have been simulated and analyzed. The results show that it is necessary to realize the reliable temperature control since the output of THz laser becomes more and more sensible for the higher pump power. In order to reduce the dimension of THz lasers, the output power of THz lasers could be ensured by appropriately increasing the pressure of working gas in THz cavity. THz output characteristics are more sensitive to the fluctuation of pump power and the drift of pump frequency for the lower power of pump laser. Consequently, the stability of pump laser should be controlled, especially for the stability of pump frequency.