Trees are regarded as natural ecological barriers against rockfall hazards. Accurate calculation of the impact force of rockfall on trees is essential for the design of ecological protection forests. However, research on rockfall impact on trees is still insufficient. A simplified dynamic model for rockfall impacting trees was established based on reasonable assumptions. The lateral forced vibration of trees under rockfall impact was analyzed through modal analysis. An analytical expression for the peak impact force was derived by combining the displacement coordination equations during the impact process with the ideal elastic model analysis method. The peak impact forces calculated by the proposed method were compared with those obtained from the classical Hertz contact theory and the ANSYS/LS-DYNA numerical simulation under specific computational conditions. The results show that the assumptions of the proposed method are reasonable. The variation trend and magnitude of the impact force obtained by the ideal elastic modal analysis method are in good agreement with the numerical simulation results which adopt an elastic constitutive model for trees. Thus, the proposed method can serve as a rapid estimation tool for the peak impact force of rockfall on trees. Result provides a scientific basis for the design of ecological protection engineering against rockfall hazards.