The breaking performance of the miniature circuit breaker with air as arc extinguishing medium is reduced, and the harm to the power system and the equipment caused by large current in case of short circuit fault is increased in high altitude environment. In order to analyze the law of environmental action in the breaking process and carry out the optimal design, the breaking process was divided into four stages according to the changing law of arc voltage in high altitude environment, the corresponding numerical equations of arc voltage and current were derived, the influence of the expected breaking current, the closing phase angle, air pressure and arc voltage on the breaking process were analyzed in this paper. The numerical equations were solved to obtain the arc current and voltage waveforms of the circuit breaker under different conditions. And the breaking test was carried out under different current and closing phase angles, it is obtained that the expected breaking current increases, the most stringent closing phase angle changes significantly in the breaking performance assessment. Then, the law of arc voltage and large current harm characteristic parameters changing with the air pressure dropping were analyzed under these phase angles. The percentages of the large current harm increase including the arc energy increase and the Joule integral increase were obtained in high altitude environment, it is obtained that the breaking performance of the circuit breaker is deteriorated obviously under low air pressure. The circuit breaker optimal design was carried out, the arc parameters were calculated numerically, and the degree of large current harm reduction after the optimized design was verified by experiments, it is obtained that large current harm characteristic parameters are reduced to the corresponding values in the low altitude area, and the proposed numerical simulation method is practical and effective.