Abstract: In order to effectively carry out the no-load and load field test of large transformer and solve the problem that the generator set and high-voltage voltage regulator used in transformer manufacturer are not suitable for substation field, a new type of large-capacity, reactive power self-compensation, and movable new test power supply device is developed, and successfully applied to the field test project of main transformer in a 220 kV station in Inner Mongolia. Application results show that the power supply device is self-compensated by reactive power, and the test voltage can meet the no-load loss test requirements of a 220 kV transformer at 1.1 times the rated voltage. By accurately adjusting the reactive power compensation effect of on-site capacitor bank, the test current can meet the load loss test requirements of a 220 kV transformer under 50% of the rated current.