When the control rod system of the third generation passive advanced PWR(CAP 1000/CAP1400)is in manual control mode,it is necessary to provide real-time,accurate and easy-to-read information about xenon distribution in the core to support its rational decision-making and proper operation.Based on the neutron physics model of CAP 1000 reactor,this paper studies the monitoring method of xenon oscillation,and develops the function of xenon mode graph in the on-line monitoring system of the third generation passive advanced pressurized water reactor core,which can effectively monitor and control the development of axial xenon oscillation.This scheme has certain applicability and can be extended to other types of PWR plants.