The propagation performance of high energy laser beams in atmosphere is studied by using extended Huygens-Fresnel diffraction integral, and expressions of axis and peak Strehl ratio are proposed. To examine the scope of these expressions, the results of wave-optics code are compared. It is found that the performance of high energy laser beams in atmosphere can be evaluated by using these expressions when the adaptive optics system is opened, but there is much difference between expressions and the results of wave-optics code under the condition of strong thermal blooming when adaptive optics system is closed.