Signal processing method for quadrant photo-detectors are investigated and how to obtain the spot parameters from quadrant photo-detectors is analyzed.Based on the research of the sources of their indices and errors for the detectors,a new calculation method on obtaining the spot parameters of quadrant photo-detector by a jiggling method is proposed.In comparison to the traditional methods,the proposed method by jiggling method can obtain not only the spot center,but also the spot radius,which can provide more relevant information different from any traditional method.In addition,this method has the advantage over the traditional method that the errors are unrelated to the spot center and the spot radius.Theoretical analysis and simulation experiments have proved that obtaining the spot parameters of quadrant photo-detector by jiggling method has considerable advantages.Field tests in the atmospheric laser photoelectric precision positioning system of inter-vehicle communications is shown that this method is advantageous in speed,accuracy and convergence.Obtained parameters are from 2 to 3 with the method.When the 30 mm GaAs-PIN quadrant detector is used,the maximum error decreases from 18% to 1.2%.