Abstract: The photo detector is an important device,which affects the detection capability in laser altimeters.In order to maintain the detector's best performance,an approximation algorithm was developed in which the approximate distribution function was used to simulate the output of the detector,so that the relationship among false detect probability,threshold and detector's gain was obtained,and the best combination of the three parameters were found.The experiment results indicate that the detector's receiver circuit based on the approximation algorithm owns apparent improvement in detect probability and echo signal-to-noise.This result is benefit for analyzing the object characteristics of laser altimeter and rebuilding the three-dimension contour of terrain and physiognomy.