The utility model provides an automatic switching device of internal -combustion engine tail gas three -way valve, the drive gear and the driven gear of setting on the three -way valve that are connected including the motor, with the motor, drive gear one side is equipped with the drive gear that closes who meshes with it, and the opposite side is equipped with the drive gear that opens who meshes with it, all be equipped with the gear on opening drive gear and closing drive gear, the gear passes through the chain with driven gear and is connected. The utility model discloses can replace electric actuator to reach provides automation the degree, improves operating efficiency, reduces personnel operational risk, reduces the scrap build and drops into, guarantees the purpose of tail gas three -way valve operation process safety, stability, quick operation.