The no-load switching of transformer will result in sympathetic inrush current of the transformer connected in series or in parallel. The sympathetic inrush current of the two transformers exhibits different characteristics from the typical inrush current, and the harmonic characteristics also change. In the AC-DC hybrid power grid, the harmonic coupling characteristic of the converter makes the harmonics in the AC power grid coupled to the DC side which will cause the DC protection malfunction and lead to DC blocking, meanwhile threaten the safe operation of power grid. In this paper, based on the parameters of actual transformer core magnetization characteristic curve, the electromagnetic transient simulation software PSCAD is used to simulate the sympathetic inrush current. The second harmonic characteristics of the single transformer inrush and the sympathetic inrush current are simulated and analyzed. The conclusion is that for single transformer the inrush current is mainly positive-sequence second harmonic current, while the sympathetic inrush current has obvious negative sequence characteristics. This will provide an analytical basis for the practical problems of the negative sequence secondary harmonics that will affect the DC protection.