In order to meet the needs of leakage monitoring in special environment such as electromagnetic interference, this paper designs an optical fiber lateral coupling leakage monitoring system based on STM32. The system is mainly divided into hardware module and upper computer module. The system hardware consists of optical fiber sensor module and signal converter module. The optical fiber sensor module is composed of POF fiber with defect structure, LED light strip and sensor package shell; STM32 is used as the main control chip in the signal converter module to realize the function of weak light signal acquisition, amplification and transmission. The upper computer of the system uses the filtering algorithm to realize the mean difference comparison of the leakage signal, filter out the occasional pulse, reduce the false alarm rate and system response time, and improve the sensitivity of the sensing band. The experimental results show that the system can work stably, each sensor point can achieve a single point calibration, the leakage monitoring range can reach 8m, and the leakage and false detection rate is less than 2%.