Abstract: A neutronics calculation code named FENNEL-N was developed based on the advanced lattice code HELIOS and the core-wise nodal diffusion code SIXTUS. Code accuracy for supercritical water-cooled reactor (SCWR) calculation was validated by comparing with Monte Carlo code. The results of assembly calculation show that the difference between PWR spectrum and SCWR spectrum is the main reason of calculation error. According to core calculations, the traditional assembly homogeneous method will introduce a large error. Studies of assembly homogenization were carried out using FENNEL-N. The results show that the change of spectrum can be taken into account by optimizing the coarse energy group structure, and the effect of reflector on assembly few group constant can be considered by employing super-cell model in assembly calculation. Using new assembly homogeneous method, the results show that FENNEL-N is suitable for SCWR pre-conceptual core design.