Sand cushions for passive protection structures could reduce the damage that is induced by rockfall impact. Therefore, evaluation of the peak impact force generated by rockfall on the sand cushion is significant to the design of passive protection structures. This study aims to estimate the peak impact force using the elastoplastic linear strengthening model when a rockfall hits the sand cushion. Impact tests were conducted to study the effect of rockfall mass, impact velocity, and cushion thickness on the rockfall impact force. The experimental results indicate that the decreasing rockfall mass, impact velocity, and increasing cushion thickness could decrease the impact force of rockfalls. The sensitivity analysis results show that the main factor that influences the peak impact force is impact velocity, followed by rockfall mass and cushion thickness. In addition, the calculation method for the peak impact force and penetration depth of rockfall was proposed by the elastoplastic linear strengthening model. The impact force-deformation curves of this model were provided and discussed. The relationship between the strengthening coefficient and influencing factors was established. In addition, the simulation results indicate that the elastoplastic linear strengthening model showed good reliability when estimating the impact force compared with the five classical models. The strengthening coefficient of other cushion materials needs to be calibrated.