In the production process of semi-finished and finished power transformers, before the newly installed transformers are put into operation, and during the operation of transformers in the power system, according to the preventive testing regulations of the Ministry of Electric Power, it is required to regularly conduct turns ratio or voltage ratio tests on the operating transformers. The traditional variable ratio bridge is cumbersome to operate, the readings are not intuitive, and necessary conversions need to be made. The test results are only one phase ratio data. The ratio tester overcomes the shortcomings of traditional ratio bridge testing. The screen adopts a one-time three-phase transformation ratio test, which is fast in testing speed and high in accuracy. Greatly saving on-site testing time and bringing high efficiency to customers' experiments.