Abstract: An important property of a supermirror is its critical angle, which is measured in multiples m of the critical angle of bulk nickel by convention. Adding Ni capping layer on the outmost of supermirrors, optimizing its key parameter ξ₀ and introducing a novel parameter to design neutron supermirrors can offset the reflectivity limitation in m≤1 region. The designed Ni/Ti neutron supermirror has less bi-layer pairs (K=200) and large m value (m=3), which would be easier to fabricate. The effects of the surface and interfacial roughness on neutron supermirrors were discussed by using Nevot-Croce model to simulate the non-ideal interface of the supermirror. The calculation result shows that 1.5 nm interfacial roughness should be required to make m=3 neutron supermirror with 200 bi-layer pairs and more than 80% critical reflectivity.