Transmission characteristics is an imp ortant index to measure the photo-detectors.The scattering parameter (S-parame ter) can be used to precisely describe the transmission characteristics of photo -detectors.In this paper,aiming at the problems of the existing test methods and systems,the microwave network analysis technology is extended to the optical wave network.Based on the principle of light wave component analy zer and the test error of photo-detectors,a new transmission characteristics te st method of photo-detectors is proposed.Also the test error model and data flo w diagram of th e photo-detectors are constructed and the S-parameter formula of the photo-de tector is derived.Based on the characteristics of photodetector,the test process is optimized and diveded into two steps;microwace domain calibratio n and optical domain test.A SOLT (short-open-load-transmission) method based on 12-term error is used in the microwave domain calibration and then the photo -detector is measured in the optical domain.The S parameter of the photo-detec tor is measured in the range from 10MHz to 40GHz.Experimental results demonstr ate that the measured data of the proposed method agreed very well with those giv en by N4373D,especially in the high frequency range,the maxinum deviation range is less than 0.5dB,which is an effective method for measuring S-parameters of photo-detectors.