With the construction of power grid intelligence and digital power grid, an electronic transformer is one of the key equipment, and its performance stability and reliability are crucial to the safe and stable operation of the power system. Due to the poor operational reliability of electronic transformers, the difficulty of operation and maintenance, and the limited performance testing methods, it is necessary to monitor the operational status of electronic transformers, and the traditional monitoring methods have been unable to meet the needs of the development. In this paper, we designed an online monitoring system for electronic transformers using voltage and electric field sensors and a high-speed high-capacity data acquisition system and verified its online monitoring function through actual testing under the true-type test platform.