Passive ductile protective system is widely used because of its better performance than buttress wall. This paper intends to determine the sliding fall rock impact force against a steel column at first. Then the rigid protective structure and ductile protective structure were compared. At last the hidden principle that passive ductile protective system can reduce the impact force was revealed. Results indicate that rings for energy dissipation in passive ductile protective system can significantly decrease the bending stiffness EI and decrease the impact force. Buttress wall encounters a lot more impact force than passive ductile protective system because of its bulkiness. Thus to ensure the buttress wall non-destructive, the thickness of buttress wall increases and it is not economic.