Abstract: In order to study the mechanical performance of UHPC under triaxial compression, the conventional triaxial experiment was carried out to analyse the mechanical properties such as the failure modes, stress-strain curve, peak stress and strain by considering the confining pressure and steel fiber content as experimental parameters. The results show that: the specimens without confining pressure and steel fiber exhibit splitting failure, while the others exhibit shear failure. The elastic modulus and elastic segment curve shape of the stress-strain curves remain unchanged as confining pressure and steel fiber content change. With the increase of confining pressure, the peak stress and strain increase continuously. With the increase of steel fiber content, the peak stress increases firstly and then remains unchanged and the axial peak strain increases firstly and then decreases, while the hoop peak strain increases. Based on the Drucker-Prager two parameter criterion, the octahedron failure criterion calculation method of UHPC was established by analyzing its octahedron normal stress, volume strain relationship and, its octahedron shear stress and shear strain relationship.