The invention provides an upper-and-lower door interlocking device at back portion of a circuit breaker switchgear. The upper-and-lower door interlocking device settles the problems of low reliability and complicated structure in interlocking an upper door and a lower door in an existing switchgear. The upper-and-lower door interlocking device comprises a base plate, a sliding plate and a first resetting member. The base plate is provided with a first interlocking plate and a second interlocking plate. The sliding plate is provided with a first interlocking hole for insertion of the first interlocking plate. The sliding plate is provided with a first position which blocks the second interlocking plate and performs spacing interlocking on the second interlocking plate in mounting, a second position which is driven by the first interlocking plate for sliding along the base plate and unlocks the second interlocking plate on the condition that the upper door is closed, and a third position which is driven by the second interlocking plate for sliding along the base plate and then is interlocked with the first interlocking plate on the condition that the lower door is closed. The upper-and-lower door interlocking device is beneficial in that the interlocking device is arranged at the back portion of the switchgear; interlocking between the upper door and the lower door of the switchgear is realized through matching between the sliding plate and the first interlocking plate and the second interlocking plate; simple structure and high reliability in interlocking are realized; and furthermore misoperation of personnel is prevented in maintenance of the circuit breaker.