A multi-channel Phase Diversity(PD) processing method is designed aiming at some practical problems of the no-common path static aberration calibration in an Adaptive Optics (AO) system. The method could be used to measure the aberration between the first image plane and the imaging CCD focused plane without changing the optical path. Compared with a traditional double-channel PD detector, the multi-channel PD method has stronger restriction to the calculation of wave front. So it has more endurance for the form of object source, much accuracy for wavefront calculation and more measuring ranges. The method has been used to measure the no-common path aberration of AO system in a 1.23 m telescope, and a good measuring result is obtained. The obtained result is taken as the offset of a Deformable Mirror(DM) to redress the static aberration, and the image resolution becomes much better.