The invention discloses a car frame longitudinal beam mold block replacing mechanism. The mechanism comprises an upper mold and a lower mold which are mounted in a cooperated manner, the lower mold comprises a lower mold base and a lower mold positioning plate arranged on the upper surface of the lower mold base in parallel, supporting positioning mechanisms are symmetrically arranged on the leftend and the right end of the lower mold positioning plate correspondingly, the upper mold comprises an upper mold base and an upper mold base plate which is arranged on the bottom of the upper mold base in the linear direction, an upper mold positioning plate is horizontally arranged on the bottom of the upper mold base plate, and a mounting plate is arranged on the bottom face of the upper mold positioning plate. The rebound phenomenon appearing during upper mold and lower mold pressing molding can be avoided, the product yield is improved, the service life of the product is prolonged, no deviation exists in the forming position of a longitudinal beam obtained after forming, the gap between a front cross beam and a rear cross beam is small due to beam assembly work, the quality needs aremet, production efficiency is improved, the assembling time is shortened, and work difficulty is reduced.