The effect of calcium formate/nano C-S-H (NC) composite on the early compressive strength of fly ash-cement system under steam curing was studied, and the influence mechanism was analyzed by XRD, DSC-TG, MIP, SEM and FTIR. Results show that doping calcium formate can significantly improve the early compressive strength of fly ash-cement system under steam curing, and the effect is the best when the dosage is 1.5% (mass fraction). Calcium formate can promote the hydration of cement and fly ash, accelerate the formation of hydration products, and reduce the porosity and total pore volume of the fly ash-cement system. Incorporating NC on the basis of calcium formate can further improve the compressive strength of the system, and the strength increases with the increase of NC content. NC can further promote the formation of hydration products, improve the degree of hydration, refine the pore structure, and improve the density of the system.