The invention relates to a data acquisition method and system for a power grid energy management system (EMS). The method comprises the following steps: selecting a port on a front-end switchboard of an old EMS, setting the port as a mirroring port, mirroring a configuration port of each front-end computer of the old EMS to the mirroring port, and acquiring all network messages of source ports of the front-end computers through the mirroring port; selecting one front-end computer from a new EMS, setting the front-end computer as a network monitoring server, selecting a mirroring network card, setting the mirroring network card to be under a hybrid monitoring mode, and establishing network connection with the mirroring port; setting a network monitoring management process; configuring a network monitoring channel and a monitoring protocol plugin; collecting the network messages of the old EMS from the mirroring network card, and processing the network messages through the monitoring protocol plugin. The technology disclosed by the invention does not have any influence on the original old EMS and acquisition substation equipment, so that the data acquisition engineering progress of the new EMS is greatly accelerated, and the engineering progress of the new EMS is shortened.