A model of a thermal blooming gas in a closed horizontal tube is used to study the effects of gas pressure on laser propagation. It focuses on the variation of beam quality in far-field and the switch time of heat conduction and heat convection in gas. By numerical simulation, the distributions of gas temperature, additional phase and beam quality and drift of mass center in far-field are shown under different gas pressures and absorptions of laser energy. And it is found that the change of beam quality is decided by both of the distribution of gas temperature and the thermal-optical coefficient, and the switch time of beam quality accords with the one between the two thermal conduction mechanisms, but the switch time of drift of mass center delays. Reducing the pressure in the tube, the gas has a higher temperature, but more even distribution, and the thermal-optical coefficient of gas scales down, so the beam quality consequently improves.