Abstract: Based on the extensive investigation of small modular reactors in the world, a small modular Pb-Bi cooled reactor with nitride nuclear fuel（SMPBN） which is designed to meet the requirements for nuclear energy improvement is presented, and the neutronics characteristics of its core is analyzed in detail. The analysis concludes the following features: the bottleneck problem of the limited uranium resources in the nuclear power development can be fixed by using the spent-plutonium as the driven fuel and thorium as the fertile fuel; the using of Pu N and Th N as the fuels can improve the safety of the reactors and increase the breeding ratio; the liquid lead-bismuth selected as the coolant and reflector not only improves the ability of the natural circulation, but also enhances the reactor safety and improves the economy of neutron; the burn-up swing is almost zero in the whole lifetime due to the perfect inner breeding ability, and all of the important reactivity coefficients are negative to assure SMPBN inherent passive safety.