Si avalanche photodiode (Si-APD) usually need series capacitors in practical applications to filter out the DC component,so as to facilitate the subsequent circuit to extract and amplify the pulse signal.In addition,Si-APD often leads to its own temperature rise after absorbing laser energy,which affects the detection performance.Based on this,in this study,a theoretical model is proposed for Si-APD in an external capacitor circuit,irradiated by millisecond (ms) pulse laser.The surface temperature characteristics of the Si-APD were studied via simulations and experiments.The results show that the current in the circuit is affected by the capacitor,and the heat dissipation of the Joule heat source inside the p-n junction in the Si-APD is reduced.The temperature rise in the Si-APD with an external capacitor is smaller than that without an external capacitor,and the smaller the capacitor,the lower the surface temperature rise in the Si-APD.