The invention provides a monitoring system and a monitoring method of a megawatt battery energy storage power station. The system comprises a central monitoring module and a local monitoring module. The two modules are in communication and connection by adopting a communication network structure in which a real-time communication network and a non-real-time monitoring communication network are in parallel. The method comprises the following steps: reading related data of the battery energy storage power station through the communication networks, and performing storage and management; determining the total power demand of the battery energy storage power station; determining the power command value of each energy storage substation and issuing the power command value to each local monitoring module; calculating the power command value of each energy storage unit in the energy storage substation, and issuing the power command value to each energy storage unit; and uploading the power command value, the real-time data and the non-real-time data of each energy storage unit in the local monitoring module to the central monitoring module for unified storage and management. The system and the method can be used to satisfy the monitoring requirements on energy storage unit cluster-type real-time and fast control and mass of transmitted data, so that the health operation of batteries can be ensured so as to make the energy storage power station more safe and stable.