The transient large current during the DC circuit breaker opening process produces a high transient magnetic field, which may cause transient disturbance voltage induced at the ports of the IGBT secondary control circuit. This disturbance may threaten the normal operation of secondary control circuit. Usually, metal shielding box is used to suppress the magnetic field. In this paper, the shielding effectiveness of an aluminum shielding box used in practical engineering is simulated by CST. Based on the results, the magnetic shielding effectiveness is analyzed in both time domain and frequency domain (DC-1MHz), which is useful for guiding the design of the shielding box.