The invention relates to a battery box side wall, an assembly type assembled battery box and an expandable battery box set. The battery box side wall comprises a side wall body, an upper flange horizontally extending to the outer side is arranged on the top of the side wall body, a lower flange horizontally extending to the outer side is arranged at the bottom of the side wall body, and an inner flange horizontally extending to the inner side is arranged at the bottom of the side wall body. The assembly type assembled battery box comprises a rectangular base plate and four battery box side walls, wherein the four battery box side walls are all vertically arranged, and connected end to end to form a rectangular frame structure, the base plate is horizontally arranged, and four sides of the base plate are respectively welded and fixed to the bottom surfaces of the corresponding inner flanges. The battery box side wall, the assembly type assembled battery box and the expandable battery box set have the advantages that the side wall is simple and reasonable in design, can be produced in bulk, and is simple to be assembled with the base plate; and the assembled battery box is simple in structure, convenient to use, high in expandability and strong in practicability.