The work aims to determine the thickness of water sac and improve the airdrop success rate of water sac. A three-dimensional simulation model of polyurethane (TPU) water sac assembly was established based on ANSYS/LS_DYNA finite element analysis software. Three kinds of ground processes of water sac airdrop impact were simulated. The thickness of the water sac was optimized. The drop impact of the water sac assembly was analyzed when the ground was inclined. The validity of the simulation method was verified through comparison with the existing experimental data. The simulation result showed that the polyurethane (TPU) water sac with a thickness of 3 mm could resist the drop impact of the water sac assembly without damage. The impact stress region of the water sac was located in the middle of both sides of the lower water sac when it fell and impacted concrete, sandy land and grassland. Sandy land had the most obvious buffer effect on the water sac, and the stress was about 1/4 of that of concrete; When the concrete ground was inclined, the impact stress of the water sac would increase correspondingly, To ensure the safe airdrop of the water sac assembly, the ground incline angle should not exceed 12°. It is feasible and effective to analyze the drop impact of airdrop water sac by finite element simulation, which can be applied to the safety design and evaluation of airdrop water sac.