This paper mainly introduces a novel method on testing for energy efficiency measurement of transformer, the main principle is that a specified current source is in parallel in the input side of transformer and a specified voltage source in series in the output side of the transformer, so the zero loss system is constructed with transformer, current source and voltage source, In terms of the internal of the system, the sum of power which is developed by current source and voltage source is equal to the internal losses of transformer, which enlighten a new method on testing for losses of transformer, finally the energy efficiency of the transformer in practical working conditions were calculated; the method can accurately measure the energy efficiency of the transformer in practical condition, and the errors which is introduced by measuring is small.