At present, the mechanical properties of ultra-high performance concrete (UHPC) products need to be further improved, and the use of nano-C-S-H/ PCE to enhance UHPC is a feasible technical way in theory. The effect of nano-C-S-H/ PCE on the mechanical properties of UHPC under steam-cured condition was studied. The influence mechanism was analyzed by measuring hydration heat, hydration products, microstructure and pore structure. The results show that the incorporation of 3. 0% ( mass fraction) nano-C-S-H/ PCE significantly increases compressive and flexural strengths of steam-cured UHPC. The addition of nano-C-S-H/ PCE can promote the hydration in early age of UHPC and form more C-S-H gel, and increase the exothermic temperature peak of hydration and shorten time to reach the hydration temperature peak. Also, the pore structure of steam-cured UHPC can be improved effectively by adding nano-C-S-H/ PCE, and the total porosity and harmful pore proportion are the lowest at 3. 0% (mass fraction) nano-C-S-H/ PCE. This study can provide a theoretical reference for the application of nano-C-S-H/ PCE in steam-cured UHPC.