Power quality is analyzed for the poloidal field (PF) power supply system of the Experimental Advanced Superconducting Tokamak (EAST) device. It is found that the power quality is quite different when the four-quarter PF power system is working in different operation modes. The (6k±1)’th (k is the odd number) harmonics are dominant during the electricity-cost phase while the (6k±1)’th (k is the even number) harmonics exist abundantly during the electricity-generation stage. In addition, fast Fourier transform as the data analysis method is applied to process harmonics and interharmonics. The experimental results present the characteristics of waveform and of current and power, and they show the emission features of harmonics and interharmonics, which leads to the conclusion that the energy of interharmonics and spectrum leakage caused by the harmonic clustering algorithm is recovered to an integer or that the picket fence effect results in the spectrum leakage of interharmonics.