Impact model between boulder and pier was built based on the Hertz contact theory and structural mechanics. Three main influence factors of impact force, including the material properties, relative size, and structure displacement were quantized and the modified formula was presented to calculate impact force. It shows that, material correction coefficient ranges from 0.20 to 0.45, which is proportional to material yield strength Y while correlated reciprocally with the equivalent modulus of system, E. The correction coefficient of ball diameter ratio exhibits that, along with the increase of structure size, it increases stably after rapid growth. Structure displacement correction coefficient is affected by the constraint condition of pier top and impact location. The practical application of this modifier formula in Dengjigou engineering result is only 6.7% of the calculated value by Hertz elastic collision theory, and according to the outcome that, relation curve between impact force and impact velocity is relatively flat, which performs in accordance with practical estimation.