For gravity caisson quay under hard bedrock conditions，riprap prisms are usually installed behind the caissons to reduce the width of the caissons，and reduce the amount of reef blasting within the excavation range of the foundation trench.This article relies on an overseas project to analyze and study the relationship between the height of the prism and the engineering cost，so as to explore the optimal design rules of the riprap prism.The results show that the engineering cost is the lowest when the height of the riprap prism is about 60%~80% of the height of the caisson.When the hard rock surface is lower than 1/2 of the height of the caisson，the optimal height of the riprap prism is about 60% of the height of the caisson.When the hard rock surface is 1/2 higher than the height of the caisson，the optimal height of the riprap prism is about 80% of the height of the caisson.The laws revealed in this article can be used as a reference for similar projects.In practical projects，multiple factors such as engineering geological conditions，ground material prices，and construction capacity should be comprehensively considered to analyze and determine an economically reasonable prism height.