Abstract: Helical coil steam generator has the advantages of compact structure, high heat transfer efficiency, strong resistance to thermal expansion stress, and has been widely applied in energy and power engineering, chemical industry and other fields. As the energy transfer facility between firstsecond loops and the isolation barrier for radioactive working medium, the inner structure of helical coil steam generator is extremely complicated, hence its structural integrity and operation reliability are necessary guarantee for the nuclear power plant safety. In this paper, the porous media method was adopted to simplify the complex structure of the multilayer helical coil domain in heat exchange assemblies, a shell side working fluid flow heat transfer characteristics analysis model was established, and the tube side watersteam two phase flow boiling heat transfer characteristics analysis model was established based on the homogeneous flow hypothesis. The gridnode data mapping method was applied to realize the coupled heat transfer simulation between shell side and tube side, and a three-dimensional full-scale thermalhydraulic characteristics analysis code HeTAF for the helical coil steam generator was developed based on the open source CFD platform OpenFOAM. The validation was performed against flow boiling heat transfer experiment of helical tube by Santini, and the calculation results of heat transfer coefficient variation with quantity are in good agreement with the measured data, indicating that the adopted model package are capable for accurate prediction under high mass flux rate and heat flux experimental condition. The helical coil oncethrough steam generator in high temperature gas cooled reactorpebble module was selected as the research object. The simulation of single heat transfer assembly under full power operating condition was performed using HeTAF. The distributions of thermalhydraulic parameters including tube side and shell side fluid temperatures, heat transfer coefficient, heat transfer power, void fraction and equilibrium quantity along tube length were obtained, and the transition process of each phase state and dominant heat transfer mechanism in the helical tube was predicted. The calculation results of helium and steam outlet temperature are fixed well with the design value with absolute errors of 5.29 K and 5.76 K respectively, which illustrates that the HeTAF can effectively predict the thermalhydraulic characteristics of both shell side and tube side of single heat transfer assembly. The work in this paper has significance reference for the design and safety analysis of helical coil steam generator. In the future, based on the existing methodology and simulation strategy, the fullscale coupled thermalhydraulic characteristics analysis of the helical coil steam generator will be conducted, and supplementary validation and subsequent optimization of HeTAF code will be carried out continuously.