In order to improve the comprehensive utilization rate of fly ash and reduce the cost of raw materials, ultra-high performance concrete (UHPC) was prepared by the mass fraction of raw fly ash without grinding and sorting instead of silica fume. The effects of different amounts of raw fly ash on the mechanical properties and microstructure of UHPC were studied. The results show that the particle size distribution of cementitious material in UHPC is gradient, and good micro-grading can be formed by the addition of raw fly ash. The fluidity of fresh concrete increases and the distribution of steel fiber in UHPC matrix is affected. When the raw fly ash content does not exceed 30%, the flexural strength of UHPC increases in varying degrees with the increase of raw fly ash content. Compared with the blank sample without fly ash, the flexural strength of UHPC with 30% raw fly ash increases by 34%. In the range of 10% to 40% content of raw fly ash, due to the slow hydration of raw fly ash, the UHPC compressive strength decreases with the increase of replacement content of raw fly ash. The pore structure analysis shows that the average pore size and total pore volume of UHPC decrease with the addition of raw fly ash, and the matrix is more dense. When the raw fly ash content is 30%, SEM images show that the steel fiber is closely combined with UHPC matrix and the interfacial adhesion is enhanced.