Abstract: With the increase of train speed, passengers' requirements for trains are no longer limited to economy and speed, and new demands are placed on train comfort and stationarity.The paper designed a real-time online monitoring system for train stationarity.According to the GB/T 5599-1985 stationarity calculation method, the monitoring system used the three-way acceleration sensor to collect the lateral, vertical and longitudinal accelerations signals of the train body, took STM32 as the core processor, completed the stationarity related calculation and analysis, obtained the stationarity value and alarm information, and uploaded the alarm information to the train control monitor system (TCMS) through the MVB (multifunction vehicle bus). In addition, the original data and characteristic data in the monitoring process are stored for later maintenance. Through the real vehicle test, the stationarity index obtained by the monitoring system and the stationarity index obtained by the computer are compared and analyzed to verify the correctness of the system.