The impact force of rockfall obtained by Hertz elastic collision theory is far more than the actual situation, and then it can not be applied in engineering practice. A modified equation for the impact force of rollfall on structures was built based on Hertz contact theory and the Thornton elastoplasticity hypothesis. The modified equation shows material properties, relative size of constrcution and structural change. Finally the modified equation is applied to anchored slide-resistant pile in 1# landslide control project of Erlang Mountain. The study shows that the calculation results of the impact force of the rollfall show that the correction coefficient is between 0.06-0.11 in 1# landslide control project of Erlang Mountain, and the calculated results are in good agreement with the actual estimation. With the increase of the impact velocity the correction coefficient decreases sharply in the early stage and decreases smoothly after. The impact force increases with the increase of the strength of the rock and the speed of the rollfall.