This paper reports a new method to compensate the wavef ront distortion. Taking a laser beam of astigmatic aberration as an example, the state of the laser beam compensating system can be given by the axial hologram of the laser beam. And the techniques of double-slit filter and "cross-filter" can be used to extract the near field wavef ront of the laser beam with phase modulation and amplitude fluctuation. It is possible to use the active compensating technique of laser beam in a high power Nd-glass laser system.