The invention relates to a mechanical interlocking safety system of a mill. The mill comprises a vacuum circuit breaker and a slow drive clutch. A safety lock A is mounted on the vacuum circuit breaker. A safety lock B is mounted on the slow drive clutch. The safety lock A and the safety lock B share a key A. When a lock column of the safety lock A and a lock column of the safety lock B stretch out, the key A can be pulled out. When the lock column of the safety lock A and the lock column of the safety lock B retract, the key A cannot be pulled out. The operation sequence of key unlocking is controlled, and therefore a mechanical interlocking relation is formed between turning on and off of the vacuum circuit breaker and disengagement and engagement of the slow drive clutch. According to the mechanical interlocking safety system of the mill, safety design is improved, misoperation can be avoided, safety risks are greatly reduced, and equipment damage or casualties caused by the confliction of two work manners of normal drive and slow drive can be reduced and even eradicated.