Objective This paper discusses the influence of parameters of the discharge circuit, including total capacitance (C), discharge voltage (U), coil inductance (L) and resistance (R) on coil currents in transcranial magnetic stimulation (TMS). Results in this paper provide theoretical support to optimize the parameters of the discharge circuits in TMS. Methods First, the paper presents the theoretically analysis of basic circuit of TMS system, and obtains the relationship between coil currents and circuit parameters. Second, the effects of circuit parameters on coil currents are investigated by simulation and experiment. The characteristics of coil currents in frequency domain are analyzed by using a Fourier transform. Results With the increasing of the capacitance of storage capacitor, the amplitude of coil current increases. At the same time, the rising time and the duration width of current pulse are extended, and the dominant frequency of current pulse decreases. With the decreasing of the total resistance of discharge circuit, the amplitude of pulse current increases, the frequency of current pulse is improved, and the current pulse shows the characteristics of multiple oscillations. With the increasing of the inductance value of discharge circuit, the amplitude of current pulse decreases, the rising time of current pulse and duration of pulse width increase, and the dominant frequency of current pulse increases to a maximum value and then decreases. Conclusions Since the characteristics of current pulse in stimulation coil are influenced greatly by different parameters of discharged circuits, the values of C, U, L and R should be matched in the practical TMS applications. Results in this paper provide theoretical reference for the design of the TMS system with specifically defined requirements.