The work aims to study the dynamic impact properties of starch/polypropylene (PP) foam cushioning packaging material under different initial strain rates and relative humidity and construct the dynamic constitutive model based on humidity and strain rate. The stress-strain curves were obtained through the dynamic impact experiments on starch/PP foam cushioning packaging material by impact test machine under different initial strain rates and relative humidity and the dynamic constitutive model was constructed. The effect of strain rate was significant under dynamic impact and the stress and energy absorption increased obviously as the initial strain rate increased. The material stress increased by 36.1% and 50.4% and the energy absorption increased by 25.8% and 36.4% as the initial strain rate increased from 30 s−1 to 34.6 and 38.6 s−1 when the strain was 0.6 under RH50%. The material was sensitive to humidity. The dynamic impact properties declined as the relative humidity increased. The stress decreased by 9.7% and 11.3% as the humidity increased from 50% to 70% and 90% when the strain was 0.6 under initial strain rate of 38.6 s−1. Based on the above analysis, the dynamic impact constitutive model of starch/PP foam cushioning packaging material was constructed based on initial strain rate and relative humidity. The initial strain rate and humidity have a certain impact on the cushioning performance of the material. The dynamic impact constitutive model based on the initial strain rate and relative humidity is verified by experiments. The experimental data are in good agreement with the constitutive model, and the constitutive model can be used to predict the dynamic impact stress-strain curve of the material.