Based on the Lagrange-Euler coupling algorithm of Abaqus software， the free drop impact process of a double-layer polyurethane/polyethylene water bag at an altitude of 50 m is numerically simulated， and the thickness of the water bag is parameterized analyzed. In specific， the transient stress， displacement and damage characteristics of the water bag under horizontal， vertical， and 45° directions hitting ground are studied. The results show that 2 mm polyethylene / 0.5 mm polyurethane layer configuration can resist horizontal drop but cannot resist vertical drop. Severe stretching from water to the bag is the main cause for the rupture. Further optimization find that 4 mm polyethylene/0.5 mm polyurethane layer can resist the impact of the water bag no matter it is under horizontal， vertical or 45° posture， ensuring the safety of the water bag. The above results indicate that the proposed method can be applied to the parametric design of liquid and solid food packaging materials.