The invention provides an overpressure preventing device for a pump box of an air separation plant. The overpressure preventing device for the pump box of the air separation plant mainly comprises a cylinder, a flange, a blank flange, a seal ring, bolts, a connection piece, a waterproof cylinder and a waterproof cover plate. One end of the cylinder is connected with the flange in a welded manner.The flange is connected with the blank flange through the bolts. The large end of the bolt is in spot welding with the flange. The blank flange can move in the vertical direction along the bolt. The nitrile butadiene rubber seal ring is arranged between the flange and the blank flange in a padding manner and guarantees that the pump box is sealed in the normal state. The waterproof cover plate isadditionally arranged outside the blank flange. The waterproof cover plate is welded to the waterproof cylinder. The waterproof cover plate, the waterproof cylinder and the connection piece constitutea waterproof assembly. The waterproof assembly is connected with the flange in a welded manner through the connection piece, and therefore it can be guaranteed that rainwater cannot enter the pump box. The overpressure preventing device for the pump box of the air separation plant has the advantages of being simple in structure, safe, reliable, reasonable, efficient, high in practicability and the like, the overpressure preventing device is suitable for small-sized isolating boxes achieving heat preservation with pearlite, such as the pump box, a valve box and a collection box, and the situation that seal gas in the isolating boxes is at overpressure, so that safety accidents are caused can be effectively prevented.