In this paper, a recently developed backward extrusion process enforcing severe plastic deformation, annular channel angular extrusion (ACAE), was successfully applied to produce the 2A12 aluminum (Al) alloy cabin. The effects of the extrusion and heat treatment on microstructure evolution and mechanical properties of 2A12 aluminum alloy were experimentally studied. Microstructure analysis and fracture surface characterization were performed to discern the reason of mechanical properties improvement via the extrusion and heat treatment, and also to explain the mechanisms underlying the grain refinement. The results showed that the grain refinement mechanisms of 2A12 Al alloy were strain-induced boundary migration (SIBM) and continuous dynamic recrystallization (CDRX) caused by severe plastic deformation. Specifically, the billet was subjected to severe shear deformation in the corner region of the ACAE, producing a large number of dislocations and promoting the recrystallization process. After the solution treatment, the alloy endured static recovery and recrystallization, and the microstructure was further refined. However, abnormal grain growth was identified after solution treatment at 495 °C for 2 h, which affected the mechanical behavior negatively. A lot of S phases precipitated from the alloy after solution and aging treatment, and the mechanical properties of the alloy were obviously improved. The specimens cut from the extruded cabin after solution (495 °C + 1 h) and aging (195 °C + 6 h) heat treatment achieved the highest ultimate tensile strength of ~476.1 MPa with an appropriate elongation to failure of ~12.9%.