This paper proposes a three-dimensional biomechanical model of the upper body and analyzes the interaction between the upper body and aircraft seat backrest for different sitting postures and backrest recline angles. The reclined sitting postures of the upper body are defined based on the available spine biomechanical data and the multibody inverse kinematics method. The contact loadings on each contacted spine segment are calculated based on the Newton-Euler dynamic formulation. The backrest contact pressure distribution is simulated using the contact stress theory based on the calculated forces on the backrest. The resultant force and pressure distribution show how the backrest inclination and lateral bending of the trunk affect the backrest loading and contact condition. The simulation results are compared to the experimental measurement for validation, and a good correspondence is found. The parameters, including the average and maximum pressure, and pressure standard deviation based on the pressure distribution, are also compared, and the maximum simulation error is 11.5% on the average pressure. Limitations of the model are discussed. The model proposed in this paper can analyze more posture cases than previous studies that focused on the two-dimensional scenarios. The loading and pressure prediction model can be applied for backrest design evaluation and facilitate seat design optimization.