Abstract: Prismatic high temperature gas-cooled reactor is one of the Generation IV nuclear reactors,and has the advantages of inherent safety and core stability.Traditional thermal-fluid calculation method of this kind of reactor is the CFD simulation,but has low computational efficiency due to the large amount of the model grid.Besides,the porous media method adopted by the pebble-bed high temperature reactor is not applicable to the prismatic core.Therefore,this paper has proposed the simplified thermal-fluid model,1D fluid and 3D solid,and has also developed the PHEASANT code based on the MOOSE platform to implement this simplified model.This paper has calculated the single channel and assembly cases of the prismatic high temperature benchmark MHTGR-350MW,and compared results with that of the CFD tool OpenFOAM.The average and maximum deviations between two codes are 1% and 3%,respectively.Besides,the calculation time of the PHEASANT code is reduced by 90% compared with that of OpenFOAM.Thus,comparison results demonstrate that the proposed simplified thermal-fluid model is accurate and highly efficient.