The three-phase combination transformer is a strong inspection and metering device, and the accuracy of its gateway measurement is very important for fair settlement in the urban power grid trade. However, so far, the General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China (AQSIQ) has not issued a special verification protocol for three-phase combination transformers. Therefore, in the error calibration test of three-phase combination transformers, the testers can only use the phase transformers of the combination transformer. Considered as independent individuals, and then individually perform error checking for each phase element in batches according to the relevant provisions in JJG314-2010 "Voltage Transformer for Measurement" and JJG313-2010 "Current Transformer for Measurement". . In comparison, ignoring the error calibration data obtained from the electromagnetic influences between the phase transformers of the combined transformer does not have a high accuracy. Therefore, a test power source capable of implementing simultaneous error checking on three-phase components has been developed. It is of great significance. This paper takes a 10kV three-phase composite transformer as the object to be measured, and designs a test power supply for three-phase combined transformer calibration, including a three-phase voltage source and a three-phase current source. The three-phase combination transformer is tested by the test power supply. The error check can choose single-phase power output or three-phase simultaneous output and analyze the practicability and reliability of the test power supply's performance test data. The test results show that the designed test power supply for the three-phase combined transformer can meet the test method requirements of the three-phase method and the single-phase method at the same time. The power output range is wide and the adjustment accuracy is high.