The invention relates to the technical field of automobile assembly, and particularly relates to a rear longitudinal beam assembly for fixing a torsion beam. The assembly comprises a longitudinal beam, a supporting piece, a top cover plate, a side sealing plate, a plurality of adjusting supports and projection welded nuts, wherein the longitudinal beam is arranged at the section, close to the vehicle tail, of the bottom of a vehicle body, the longitudinal beam is a J-shaped face plate and is provided with an mounting flat face plate and a vertical curved face plate which are integrally formed,the side sealing plate is fixedly connected with the mounting flat face plate and the top cover plate to form a cavity for containing the supporting piece, a plurality of through holes are formed inthe top cover plate, a plurality of limiting holes are formed in the peripheries of the through holes, the adjusting supports are provided with containing holes, all corners of the supporting piece are provided with channels, and the longitudinal beam is provided with a plurality of mounting holes. According to the assembly, the rigidity and modality of a mounting part are improved, and by adjusting the movement amount of the supports, the accumulated tolerance of peripheral parts can be effectively absorbed, the number of the times of reworking are reduced, and the assembling efficiency is guaranteed.