Based on the ±800 kV 4-terminal LCC-HVDC model of parallel ring network topology, the structural parameters and action timing of the DC circuit breaker are given. The electromagnetic transient of the DC circuit breaker during the breaking process is analyzed for the single pole ground fault condition. The characteristics and the current distribution of the DC circuit breaker after the fault line is cut off, the voltage of the energy-absorbing arrester and the absorbed energy, and the influence of the fault line cutting on the system operation are studied. The effects of current-limiting inductor, breaking time of DC circuit breaker and the delay of phase-shift on the electromagnetic transient characteristics of DC circuit breaker breaking process are pointed out.