Abstract: The NECP-SARAX code developed by NECP research team is used for the fast reactor analysis. In order to consider the complex neutron slowing down effects from scattering resonance of nuclides with medium mass and inelastic scattering, the Monte-Carlo method was used previously but brought big uncertainty in tallying the spectrum in low energy. Besides, the efficiency is extremely low. In the improvement, a new resonance method based on ultra-fine group and point-wise cross section was involved. It generated effective multi-group cross sections with similar accuracy of Monte-Carlo method but avoided its disadvantages. In the core calculation, the multi-group cross sections were used for the 3D transport calculation. The first-order and exact perturbation theories were used in calculating the reactivity coefficients of fast reactor. To be capable of handling the geometric distorting in fast reactor, a new pseudo density method based on neutron transport and perturbation theory was proposed. Both the point kinetic and quasi-steady method were used in the transient calculation as two options to fit the requirement of fast reactor and ADS. The OECD fast reactor benchmarks were tested to verify the SARAX code, the results show good accuracy in the fast reactor calculations, which is similar to other widely accepted fast reactor code. The relative error of keff is less than 300 pcm compared with the Monte-Carlo code. Based on the pseudo density model and three-dimensional spatially dependent kinetics model, the SARAX code has more complete functions to fit the requirements of fast reactor design better.