Partial discharge will continuously corrode the insulation layer of cable, which will greatly reduce its insulation performance, thus causing cable operation failure and power failure. For some overhead lines close to buildings, these problems will bring great security risks to the lives and property of residents. By monitoring the partial discharge phenomenon of cable intermediate joint, the insulation condition of indirect joint can be judged in time, and the insulation defect can be found early, thus reducing the cable failure rate of distribution network. In this article, the monitoring algorithm of partial discharge in transmission cable based on optical fiber sensing technology is studied. By detecting the interference signals of adjacent weak fiber gratings, the frequency, phase and position of acoustic vibration signals of partial discharge in the middle joint of cable are obtained, so as to judge the strength and position of partial discharge. The simulation results show that the algorithm can demodulate the partial discharge signal of each cable intermediate joint accurately and quickly, and realize large-scale distributed real-time online monitoring. The algorithm can demodulate the partial discharge signal of each cable intermediate joint accurately and quickly, and realize large-scale distributed real-time online monitoring.