There is a close relation between flicker and inter-harmonics. This article further reveals the quantitative relationship between them from the perspective of signal modulation theory. On the one hand, considering the phase parameter influence, analyzes the amplitude modulation and phase modulation effect when the inter-harmonic superposed on the power frequency signal, and concludes that only when the amplitude modulation occurs, the inter-harmonic causes the signal envelope fluctuation. On the other hand, the quantitative calculation formula of the inter-harmonic parameters caused by amplitude modulation wave is deduced from the flicker signal mathematical model. Smulations based on the Matlab software verify the correctness of the conclusions.