Plenty of harmonic and inter-harmonic in the utility grid injected by fluctuating loads strongly impact on the safe and stable operation of power grid. According to GB/T 17626.30, there exist three kinds of measurement methods, namely class A、S and B, for each parameter measured, however they are adopted by different instruments leading to difference between measuring result in domestic power quality test instruments which become a long-standing issue. Based on the definition and algorithm of the three measurement methods of Class A、S and B, this paper analyzes and compares the effects of the three measuring methods on harmonic analysis by their maximum value and 95% probability maximum value separately with the measured data of two typical fluctuating loads spot welder and EAF. Finally from the perspective of practical engineering application, it proposes the applications and relevant conclusions for the three measuring methods respectively.