This paper introduces the system composition and working principle of the battery on-line monitoring system applied in the fully automatic operation of metro train, and expounds the estimation method of SOC of the remaining battery in detail. This system can not only effectively use and manage the energy of the vehicle battery, ensure the battery to maintain within the normal working range, extend the service life of the vehicle battery, but also upload the test parameters to the cloud server through Ethernet, establish a large database, train the battery model repeatedly, which is used to analyze the maintenance and operation of the battery and the performance status of the battery Make early prediction of health status to ensure the safety of battery.