The frequency stability of the terahertz (THz) radiation source is one of the key issues which directly affect its application regarding to THz imaging, Radar detection, Coherent communication and so on. Based on two-photon light shift effect, the physical model for the frequency shift of optically pumped THz lasers has been built up and the analytical formula for calculating the frequency shift has been derived. Taking the optically pumped methanol(CH3OH) for example, the absorption line of methanol under different pressure has been given, the influence of the bump laser frequency offset and the power of pump laser on the frequency stability of output THz laser have been analyzed quantitatively while the effect of the pressure in THz cavity on the THz laser frequency drift has also been discussed. The results show that THz laser frequency drift increases with the increasing of the bump power, whereas it decreases with the increasing of pressure in THz cavity. The maximum of THz laser frequency drift reaches under the condition that the bump laser frequency drift is equal to a quarter of the absorbed line width of the operating gas when the bump laser frequency offset locates within a certain range. In practice, in order to improve the output frequency stability of THz laser, not only the appropriate working conditions (pressure, temperature) should be chosen, but also the measure for controlling the bump laser frequency shift within a certain range should be taken.