Because of its excellent insulating properties and environmental performance, the perfluoroisobutyronitrile (C4F7N) gas mixture is the most promising alternative to greenhouse insulating gas sulfur hexafluoride (SF6). However, after multiple arc extinguishing processes, various decomposition residual products will be produced, and the impact on the safety of maintenance personnel is still unknown. Therefore, this paper systematically evaluated the biological safety of C4F7N/Air decomposition residual products. The results demonstrated that the decomposed C4F7N/Air gas exhibits substantial acute toxicity when inhaled with an LC50 value of 31.21 parts per million for male lab mice (four hours), significantly lower than that of pure C4F7N gas. As a result, personnel involved in the operation and maintenance of electrical equipment must strictly adhere to safety precautions.