The invention discloses a protocol communication method and system for online acquisition of voltage transformer operation data. The method comprises the following steps: distributing communication addresses for a plurality of groups of voltage transformers; according to DLT07-645 extension protocol, sending a data inquiry terminal request frame of the voltage transformer corresponding to the communication address to an online monitoring device through a terminal; according to the DLT07-645 extension protocol, receiving the data inquiry terminal request frame through the online monitoring device, encapsulating the monitored data of the voltage transformer into a data device response frame, and sending the data device response frame to the terminal; according to DLT1376.1 extension protocol, sending a data master station request frame for inquiring the data of the plurality of groups of mutual inductors to the terminal by using a master station; and according to the DLT1376.1 extensionprotocol, encapsulating the received data of the plurality of groups of voltage transformers into a data terminal response frame through the terminal, and sending the data terminal response frame to adata acquisition master station.