With the extensive use of all-electric aircraft, how to achieve lightweight motor and improve power density to meet higher performance requirements has gradually become a hot research issue. An outer rotor surface permanent magnet synchronous motor (SPMSM) used in all-electric aircraft is taken as the research object. Based on finite element simulation, the static mechanical field of 6063t6 aluminum alloy and steel is analyzed, and the lighter 6063t6 aluminum alloy is selected. Under the premise of safety factor of 10, end cover thickness is reduced to 2 mm and shell thickness is 5 mm. Then, a hollow-out casing structure is proposed, and with the “crab clamp” stiffener, the safety factor of the proposed structure is successfully improved from 5.5 to 10, and the weight of optimized casing is reduced by 51.92%. By comparing the structure of the “crab clamp” type with linear type stiffeners, the superiority of “crab clamp” type stiffener is verified. The optimized motor structure meets both lightweight design and reliable operation requirements.