The advanced autonomous driving system enables more flexible seating arrangements. While a reclined seat can enhance comfort during the ride, it may also significantly affect kinematic response of the occupants. Currently, there are two common virtual tools for studying occupant injury:​ Human Body Models and Anthropomorphic Test Device Models. The differences in the internal structural responses of these two groups under reclined postures are not well understood. This study focuses on analyzing the kinematic and mechanical response of occupants' pelvis and lumbar spine under rigid seat with zero gravity characteristics. Four seat conditions were designed, altering angle of seat‐back, seat pan, and leg rest. Simulations were conducted using THUMS v6.1 and THOR‐AV model for highly reclined angle research, subjecting them to a 47 km/h frontal crash pulse on a sled. The study found that as the recline angle increases, the forward movement of the pelvisincreased, which led to increased interaction forces between the pelvis and lap belt, and increased compression forces on the lumbar spine. In addition, raising the seat pan could help reduce the forces on the pelvis and lumbar spine. THOR‐AV and THUMS align well in pelvic forward motion but show clear differences in pelvic rotation. There were also some differences in the degree of Z‐direction movement of the head and torso. In particular, THUMS exhibits lumbar extension moments at high reclined angles, necessitating further research on the load‐bearing and injury mechanisms of the occupants' lumbar spine in high reclined postures.