This paper addressed the safety issue of occupants during aircraft crashes by constructing a multi-scale finite element (FE) model that included the Hybrid III anthropomorphic test device (ATD) and biomechanical spine, achieving the impact dynamic response of the occupant's entire body and the fine anatomical structure of the spine during aircraft crashes. Firstly, a FE model of the seat-occupant dummy coupling system was established in LSDYNA software, with the vertical impact test conditions of seats defined in AAC Part 29 as the load input. The impact dynamic response of the seat-occupant coupling system was analyzed based on explicit dynamics. Secondly, a three-dimensional nonlinear FE model containing the L1-L5 full lumbar spine biomechanical anatomical structure was constructed, and the biomechanical response of the occupant's lumbar biomechanical structure during the impact process was further analyzed based on implicit integration. The simulation results showed that the maximum compressive load on the lumbar spine of the occupant in this case was 5554 N, which is lower than the safety threshold of 1500 lb (6672) N specified by FAR25.562. The stresses of the bones and intervertebral discs given by the lumbar spine FE model were all within the safety range, and there was no injury risk in the occupant system. This study provides a safety assessment method for the crashworthiness testing of seat-occupant systems based on biomechanical finite element analysis, realizing the consideration of the fine biomechanical privacy of the spine in the safety analysis of aircraft.