Avalanche photodiode (APD) has the characteristic of detecting weak light signals. The detectioncharacteristic of APD is directly depended on the design of relevant processing circuits. The operation principle andcharacteristics of APD electro-optic detectors and external influence factors are analyzed. The composition of opera.tion circuit and the best design of APD are researched. During signal post processing, the signal amplification pro.cessing circuit of APD is designed. And the requirements of low noise, bandwidth matching and weak signal amplifi.cation are met, which has an important significance in the field of weak light signal detection of APD.