Abstract: A thermal coupling model between a medium gas and glasses in a vertical closed tube is established to study thermal effects on laser propagation. Using the thermal coupling method of gas and solid, the gas temperature distributions are simulated, which decide laser wavefront phase distributions in near field and laser beam quality in far field. It is shown that under gravity, gas temperature increase results in its flow in the tube, and the thermal coupling effects between gas and two glasses, one in the top and the other in the bottom, are important for gas temperature distribution,especially the bottom glass. Thus, laser wavefront phase distributions and beam quality change all along during laser propagation. Compared with not considering thermal coupling effects, the gas temperature distribution and beam quality in far field change much.