In this paper，both the current and the voltage in a transformer are measured by digital oscilloscope. The nonlinear relation between the current harmonic and the voltage is discussed via harmonic analysis，thereinto，the amplitude of three harmonic currents increase rapidly with the increase of the voltage，which is approximately proportional to the voltage to the third power，and the amplitude of the even harmonic current does not significant increase with the voltage. The experimental results are consistent with the theoretical analysis. In addition，three harmonic signals are obtained through different connection of winding in the three-phase transformer，and rule of the three times harmonic wave of the current changing with the voltage is studied.