By discussing intensity distribution, and Strehl's ratio before and after perfect phase compensation with a rube-mirror, the limitations of amplitude fluctuation on adaptive optics system are analysed. Although phase distortion can be corrected perfectly, there is a theoretical limit on compensation because the amplitude distortion can not be compensated. A general expression of the limit Strehl's ratio which is variated in whole region of fluctuation is derived. The theories show that the corrected peak intensity is only half of the diffraction intensity when amplitude fluctuation is maximum, the lowest value of the Strehl's ratio limit is 0.5.