The boulder impact force in debris flow is generally calculated by static methods such as the cantilever beam models. However, these methods cannot describe the dynamic scenario of boulder collision on structures, so the inertia and damping effects of the structures are not involved causing an overestimation on the boulder impact force. In order to address this issue, a dynamic-based model for calculating the boulder impact force of a debris flow was proposed in this study, and the dynamic characteristics of a cantilever beam with multiple degrees of freedom under boulder collision were investigated. By using the drop-weight method to simulate boulders within debris flow, seven experiments of drop-weight impacting the cantilever beam were used to calibrate the error of the dynamic-based model. Results indicate that the dynamic-based model is able to reconstruct the impact force history on the cantilever beam during impact time and the error of dynamic-based model is 15.3% in calculating boulder impact force, significantly outperforming the cantilever beam model's error of 285%. Therefore, the dynamic-based model can overcome the drawbacks of the static-based models and provide a more reliable theoretical foundation for the engineering design of debris flow control structures.