There are some problems in the operation state of online electric energy meters, such as monitoring blank spots and loss caused by inaccurate measurement, which will reduce the economic benefits of power grid enterprises. This paper presents an online monitoring method suitable for the medium voltage side, which determines whether the voltage at the low voltage side of the transformer changes with the load and conforms to the external characteristic law of the transformer. The measured data collected by the metering device are used to calculate the voltage value of the first three-phase line at the operation of the transformer area. The three-phase balance rate and voltage fluctuation rate are used as multi-objective functions to solve the correction coefficient of current deviation.