Abstract: High temperature reactor pebble-bed module demonstration plant (HTR-PM) consists of two reactor modules with one steam turbine system. The model of steam generator needs to be improved, although the engineering simulator system for HTR-PM was developed by Institute of Nuclear and New Energy Technology, Tsinghua University to study the operation characteristics of HTR-PM. In this paper, the flow and heat transfer characteristics of once-through steam generator were studied, and the models of the flow and heat transfer between primary and secondary fluid were also developed. The analysis of distributed data proves that the model and simulation results are both correct. The simulation accuracy was improved by optimizing the models and mesh generation on the premise of ensuring real-time calculation, so that the improved model can be applied for further study on operation characteristics of HTR plants with more reactor modules by engineering simulator.