Pointing,acquisition and tracking (PAT) system is the key component in inter-satellite laser coherent communications. A novel method to coherent detection of position errors information is proposed. Based on the system composing,this method adds CCD and computer as position error detector. The position errors as the control signal of PAT subsystem drive the receiver telescope to keep tracking to the target. Theoretical deviation of the relation equation is given,and the analysis of the effect of the wavelength stability on the precision is presented. Finally the simulation results validate the method.