Abstract: Mathematical models with modular structure are developed for primary system of LMFBR. The effective algorithm are adopted to solve these models, such as the end floating method solving stiffness of point reactor neutron kinetics equation and the fully implicit second order upwind difference solving the core thermal and the IHX. A LOHS simulation program run on microcomputer with a faster than real time simulation speed is developed, and used to calculate the loss of heat sink transience experiment of EBR Ⅱ. The calculated results coincide well with those by using NATDEMO. Finally, the inherent safety of fast reactor is analyzed according to the calculated results.