Considering the kinetic and fluid dynamic processes in the gain medium, a theoretical model is established to describe the mechanism of thermal-lensing effect in an exciplex pumped Cs vapor laser. The three-dimensional distribution of temperature and index of refraction in the gain medium are depicted. The effective focal length and radius of thermal lens are predicted. Our simulation results show the thermal lens plays a non-negligible role in high-power XPCsLs and can be significantly aggravated in higher wall temperature, buffer pressure and pump intensity. The divergence of laser beam influenced by thermal lens is also made in detail. This model is helpful for in-depth understanding of the thermal-lensing effect in XPALs.