To reduce the viscosity and promote the workability of ultra-high performance concrete (UHPC), this paper selected aminosilane (KH550) with a hydrophilic functional group as auxiliary admixtures for UHPC mixtures. The effects of the mixture ratio and dosage of hydrolyzed silane on the mechanical properties and rheology of UHPC were investigated. The silane modification mechanism of UHPC was discussed. The results show that KH550 is suitable for hydrolysis in the mixed solvent of anhydrous ethanol and deionized water (or tap water). The optimum hydrolysis ratio (mass ratio) is determined as m1 (KH550): m2 (deionized water or tap water): m3 (anhydrous ethanol)=1.0:3.0:(2.5-5.0). The hydrolysis process is carried out under stirring for 3 min and hydrolyzed for more than 2 h. The optimum silane content in UHPC (calculated according to a mass ratio with silica fume in UHPC) is 1%. The Bingham model can be used to describe the rheological behavior of fresh UHPC modified by hydrolyzed silane. The rheological properties of fresh UHPC modified by hydrolyzed silane are improved, but the effect on the mechanical properties is ignored.