The geometric dimension of steel fiber is a key factor for the reinforcement and toughening of ultra high performance concrete（UHPC）. In this paper，the working and mechanical properties of UHPC with different length diameter ratio are experimentally studied，and the influence mechanism of steel fiber length diameter ratio on UHPC is analyzed. The results show that with the increase of the length diameter ratio of steel fiber，the slump and expansion of UHPC continue to decrease and the gas content increases. With the increase of steel fiber aspect ratio，the compressive strength，flexural strength，flexural toughness coefficient and impact resistance of UHPC are increasing，but the increasing range shows a decreasing trend. The length diameter ratio of steel fiber should be controlled at 60～100，and UHPC with excellent construction and mechanical properties can be prepared. The developed UHPC has been applied in the repair and reinforcement of pile foundation of a wharf project and achieved good application results.