In order to explore the fracture performance and mechanism of ultra-high performance concrete(UHPC)，the effect of polyvinyl alcohol fiber(PVA)and polypropylene fiber(PP)replacing part of steel fibers on the fracture performance of UHPC was studied by three-point bending fracture test with the substitution rate of synthetic fiber as the variable. The modified double-K fracture toughness formula and fracture energy formula were used to calculate the fracture performance indexes of UHPC with different fiber contents. The results show that，the phenomenon of "secondary hardening" occurs in the load-CMOD curve and obvious fluctuation can be found in the descending section of the load-deflection curves after replacing part of steel fiber with synthetic fiber(PVA or PP). The initiation toughness and ductility of UHPC could be effectively improved by replacing part of steel fiber with PVA fiber，and the cohesive toughness and bridge connection toughness are decreased，as so to reduce the unstable toughness. When PP fiber is used to replace part of steel fiber，the fracture initiation toughness of UHPC is slightly decreased，but the unstable toughness and fracture energy are reduced significantly. The fracture mechanical properties of UHPC could be comprehensively improved when 30% of steel fiber is replaced by mean proportion of PVA fiber and PP fiber，which is the most ideal solution for replacing steel fiber.