An increasing number of high-speed trains are put into operation in China, whose current harmonics from grid-side converter (GSC) are introduced into the traction power supply network. However, the traditional linear time-invariant (LTI) modeling of grid-side converter makes it difficult to analyze the current harmonic characteristic and coupling phenomenon because of the power switching devices and the nonlinear controller. In this paper, the linear time-periodic (LTP) system modeling method is employed to investigate the single-phase GSC applied in CRH3 electric multiple units. The harmonic transfer function (HTF) small-signal model of the GSC is derived after local linearization and investigated to analyze the harmonic interaction in frequency domain and the steady-state response in time domain. At last, the nonlinear simulation results are represented to verify the effectiveness of the HTF harmonic analysis.