An auxiliary valve motion device of an engine comprises a double-valve actuator, a single-valve actuator and an auxiliary motion control mechanism. The double-valve actuator comprises a double-valve driving mechanism, the single-valve actuator comprises a single-valve driving mechanism, and the auxiliary motion control mechanism simultaneously controls the double-valve driving mechanism and the single-valve driving mechanism. When the auxiliary motion control mechanism is started, the double-valve driving mechanism is changed from an operating position to a non-operating position, the double-valve actuator and two valves of the engine are disconnected, and conventional valve motion of the engine is eliminated; meanwhile, the single-valve driving mechanism is changed from the non-operating position to the operating position, the single-valve actuator drives one of the two valves to generate auxiliary valve motion of the engine. Auxiliary valve motion and conventional valve motion of the engine are not interfered, the auxiliary valve motion device can be used for braking of the engine, braking loads are reduced, and braking performance is improved.