Cu$2$O nanoparticles with controllable facets are of great significance for photocatalysis. In this work, the surface termination and facet orientation of Cu$2$O nanoparticles are accurately tuned by adjusting the amount of hydroxylamine hydrochloride and surfactant. It is found that Cu$2$O nanoparticles with Cu-terminated (110) or (111) surfaces show high photocatalytic activity, while other exposed facets show poor reactivity. Density functional theory simulations confirm that sodium dodecyl sulfate surfactant can lower the surface free energy of Cu-terminated surfaces, increase the density of exposed Cu atoms at the surfaces and thus benefit the photocatalytic activity. It also shows that the poor reactivity of the Cuterminated Cu$2$O (100) surface is due to the high energy barrier of holes at the surface region.