There are some deficiencies of the IEC flickermeter used to detect flicker caused by interharmonic voltage. The effective frequency range of IEC flickermeter detection and the detection method of flicker caused by interharmonics are discussed. IEC flickermeter model and flux mathematical model are developed. The flux fluctuation frequency, flux fluctuation value and flicker value of amplitude modulation (AM) voltage and interharmonic can be obtained. The relationship between flux and flicker can be fitted on the basis of numerous data of AM voltage. The relationship is considered as a standard relationship, which is used to discuss the effective range of IEC flickermeter in regard to interharmonics. Furthermore a new approach, used to detect flicker caused by interharmonics, is presented.