The invention discloses an automatic mechanical opening device of a circuit breaker of a switch cabinet. An opening hole is formed in the switch cabinet. The automatic mechanical opening device comprises a support base, a push rod motor and an insulating rod. The push rod motor is fixed to a position by the aid of the support base, the position corresponds to the opening hole in the switch cabinet, the front end of a travel rod of the push rod motor is connected with the insulating rod, and the push rod motor is provided with a clockwise rotation circuit and an anticlockwise rotation circuit. The automatic mechanical opening device has the advantages that the clockwise rotation circuit or the anticlockwise rotation circuit of the push rod motor can be closed to drive the insulating rod to advance towards the opening hole, so that the circuit breaker can be opened or can be withdrawn from the opening hole and can be automatically mechanically opened, and operation workload on operation and maintenance staffs can be saved; time relays are serially connected with the clockwise rotation circuit of the push rod motor, so that the clockwise rotation circuit can be closed in a delayed manner, the motor can start to run for closing the circuit breaker after the operation and maintenance staffs leave the to-be-opened circuit breaker and arrive at safety distances in sufficient available time, and the personal safety of the operation and maintenance staffs can be guaranteed without shutdown of power distribution networks.