According to the experiments which concern about nonlinear loads in low voltage distribution systems,the voltage crest factor has a crucial effect on the establishment of nonlinear load harmonic Norton model,and it is verified by a large number of measured data. In the low voltage system that voltage waveform presents a flat characteristic,the nonlinear load can be simplified by the Norton model. Experimental and simulation results also show that different reference voltages can lead to different mean values of Norton model parameter which is obtained by fluctuation quantity method. The differences don’t have any effect on harmonic load flow analysis,but it affects the accuracy and credibility of the assessment load harmonic contribution level.