[Introduction] The paper aims to effectively suppress the disturbance of non characteristic harmonics to the power grid in the tokamak power supply system, the paper studies the mechanism of low-frequency harmonics in the tokamak power supply system. [Method] By introducing an improved switching function model, selected ITER fusion device as an example and established its corresponding AC/DC equivalent circuit model combined with its corresponding parameters, the corresponding output second harmonic was obtained through calculation and simulation. [Result] The ratio of output to input is calculated, and the stability factor value almost tends to zero. [Conclusion] It is determined that the DC bias of rectifier transformer do not cause harmonic instability of the system.
