This study presents an insight into the time-varying characteristic, especially for the non-sine wave characteristic, of magnetic field (MF) of the wireless power transfer system. First, a filament mode is built and a key expression of MF is obtained, presenting a theoretical proof of the existence of the non-sine wave. Second, the reasons and conditions of the non-sine wave are discussed from both electromagnetic and circuit views. Third, the quantity of MF sine-wave distortion (MFSD) is proposed to quantify the non-sine wave characteristic. The current phase difference (CPD) and the current magnitude ratio (CMR) are used to denote the working conditions of the WTPS. The MFSD under a sweeping CPD and CMR is analysed, perfectly quantifying the non-sine wave characteristic and showing that the CPD = −90° and CMR = 1 are the most evident situations of the non-sine wave appearing. Fourth, a sine-wave assumption and its related complex magnitude of the total MF are analysed and compared by the maximum real value of MF. This finding shows that an error of 35% could appear under the sine-wave assumption but can be eliminated by the method in this study. Finally, an experiment has been conducted to verify the analysis.