Abstract: In response to the hexagonal geometric characteristics of the VVER core, the conformal transformation method is adopted to transform the diffusion equation under hexagonal geometry into a diffusion equation under rectangular geometry. The transformed diffusion equation under rectangular geometry is solved using lateral integration technique: semi analytical approximation of the lateral integrated flux is performed, and parabolic approximation is introduced for the lateral leakage term and neutron source term, and the source iteration method is used for solution. Based on the above algorithm, the hexagonal geometry neutron diffusion software SANM-H was developed, and the program was numerically validated using two-dimensional and three-dimensional benchmark problems. The results indicate that using this method to solve hexagonal geometry core problems can achieve high computational accuracy.