The focus shift of laser caused by atmospheric thermal blooming is studied. An approximate computational method of thin lens group is used, which is simple and quick. The results are as follows. The effect of atmospheric thermal blooming on high power lasers is related to the initial power, the emitting aperture diameter, the propagation path and the atmospheric absorption coefficient. The focus will shift along the project direction when the absorption coefficient decreases with the increase of the propagation path, while it will shift against the project direction when the absorption coefficient increases with the increase of the propagation path.