This paper analyzes the self-mixing interference effect in the laser diode (LD) with multiple external reflectors and presents the analysis based on the compound cavity effect in lasers. Each external cavity is considered to be an optical thin film, and the reflectance ratio can be get from the theory of the thin film optics. The general expression of the threshold gain and frequency in the compound laser cavity are deduced and some simulation analysis is made at different conditions. A self-mixing interference experiment system is build with multiple external reflectors. And some experiment results are presented. It is found from the simulation and experiment that the output of the self-mixing interference system with multiple external reflectors is sinusiodal or saw-toothed waves which is similar with the three-mirror Fabry-Perot cavity system. The simulating and experimental results demonstrate the validity of the principle. This principle can be used to analyze the fiber-coupled self-mixing system and the simulation is accordant with experimental results.