Based on semiconductor physics and the essential structure of IGBT, the turn-off mechanism of IGBT is deeply discussed regarding the problem of turn-off time changing with voltage and current. The laws of turn-off time changing with voltage and current are deduced, i.e., the turn-off time increases with voltage increasing and decreases with current increasing. The physical mechanisms of the laws are found out. The simulation results and experimental results, demonstrate that the derived and the existing law are constant, thereby proving the correctness of the derived law. It is put forward that the law of IGBT turn-off time changing with current and voltage accords with the complex law of exponent and hyperbola. For further studying the IGBT turn-off mechanism and solving the engineering puzzles including the power electronic dead time setting, the present study is significant in theory and practical application.