Method to rovide emergency power supply to nuclear power plant, comprising: providing an accumulator battery system, wherein said accumulator battery system is connected to emergency bus and monitored by online monitoring system, wherein the accumulator battery system comprises a plurality of accumulator system modules in parallel connection, said plurality of accumulator system modules are connected to the emergency bus via a bus rod, each accumulator system module comprises a converter device and a battery array, said battery array is connected to the bus rod via the converter device, a first switch control unit is disposed between the accumulator system module and the bus rod, the online monitoring system controls the accumulator system module to put into operation via controlling the first switch control unit according to a load capacity of the nuclear power plant under current working conditions, wherein the online monitoring system is configured to collect performance parameters of batteries in the accumulator battery system, and calculate the total capacity of the accumulator battery system, wherein the performance parameters include capacity, voltage, current, temperature and internal resistance, monitor working conditions of the nuclear power plant, calculate the load capacity of the nuclear power plant under current working conditions, in case of power loss of the electrical devices of the nuclear power plant, which can be powered via the emergency bus, start the accumulator battery system to provide power supply to the electrical devices of the nuclear power plant via the emergency bus according to the total capacity of the accumulator battery system and the load capacity of the nuclear power plant under current working conditions, that is, put a first accumulator system module in the accumulator battery system into operation; put a second accumulator system module in the accumulator battery system into operation, and perform in loop until all accumulator system modules are put into operation or the accumulator battery system meets corresponding power supply requirements. providing an accumulator battery system, wherein said accumulator battery system is connected to emergency bus and monitored by online monitoring system, wherein the accumulator battery system comprises a plurality of accumulator system modules in parallel connection, said plurality of accumulator system modules are connected to the emergency bus via a bus rod, each accumulator system module comprises a converter device and a battery array, said battery array is connected to the bus rod via the converter device, a first switch control unit is disposed between the accumulator system module and the bus rod, the online monitoring system controls the accumulator system module to put into operation via controlling the first switch control unit according to a load capacity of the nuclear power plant under current working conditions, wherein the online monitoring system is configured to collect performance parameters of batteries in the accumulator battery system, and calculate the total capacity of the accumulator battery system, wherein the performance parameters include capacity, voltage, current, temperature and internal resistance, monitor working conditions of the nuclear power plant, calculate the load capacity of the nuclear power plant under current working conditions, in case of power loss of the electrical devices of the nuclear power plant, which can be powered via the emergency bus, start the accumulator battery system to provide power supply to the electrical devices of the nuclear power plant via the emergency bus according to the total capacity of the accumulator battery system and the load capacity of the nuclear power plant under current working conditions, that is, put a first accumulator system module in the accumulator battery system into operation; put a second accumulator system module in the accumulator battery system into operation, and perform in loop until all accumulator system modules are put into operation or the accumulator battery system meets corresponding power supply requirements.