According to the relationship between the thermal effects and absorption coefficient of the inner channel gas, reaction length and intensity distribution of the laser beam, a model of thermal effects on laser beam quality in the inner channel propagation is established based on the steady-state heat-conduction equation. The relationship among the temperature distribution, the optical path difference across the beam, RMS phase distortion, Strehl ratio in the far-field and the obscuration ratio, the axial flow velocity is calculated. The approximate calculation results of the temperature distribution are about the same as the theoretical results. The model provides a qualitative reference for the laser beam control system design and performance evaluation.