The invention discloses a lower shell of a steel-aluminum hybrid battery pack. The lower shell comprises a frame structure, cross beams, longitudinal beams, bottom plates and middle lifting lugs; themiddle lifting lugs penetrates through the cross beams and the bottom plates from top to bottom and are fixed to the cross beams and the bottom plates, and each of the middle lifting lugs comprises amiddle lifting lug body of a cylindrical hollow structure, wherein the top end of the middle lifting lug body is matched with a middle lifting lug nut, and the lower end of the middle lifting lug bodyis sequentially installed in a middle lifting lug reinforcing plate and a middle lifting lug sleeve in a penetrating mode; the frame structure is formed by splicing four frames, the four frames are divided into two groups, and each group of frames are the same in structure and are oppositely arranged; the cross beams are formed by embedding an inverted U-shaped thin-wall structure with a lightening hole into an inverted T-shaped thin-wall structure with a lightening hole, and the frame structure is made of aluminum alloy; the cross beams, the longitudinal beams, the middle lifting lugs and the bottom plates are all made of steel. According to the invention, the advantages of aluminum alloy and high-strength steel are fully absorbed, the lightweight and low-cost design of the battery packbox body is realized, the welding deformation is small, the connection strength is high, and the processing efficiency is high.