Abstract: Supercritical steam generator can further improve the power generation efficiency of high temperature gas-cooled reactor (HTGR). A supercritical steam generator design is proposed based on the experience of the subcritical once through steam generator of high temperature gas-cooled reactor-pebble bed module (HTR-PM) and the future development plan of HTGR. An one-dimensional steadystate thermalhydraulic code for the helical tube supercritical steam generator of HTGR was developed based on the thermal hydraulic code of HTRPM subcritical once through steam generator. The empirical correlations of convective heat transfer coefficients of supercritical fluids are different from that of subcritical fluids. Different empirical correlations of convective heat transfer coefficient for supercritical fluid were compared. Their influence on the thermal-hydraulic calculation results is found to be quite small due to the large thermal resistance of the primary side helium convection. The 100% load working condition and partial load working conditions were calculated using the developed code and the distributions of the parameters including temperature, velocity, convective heat transfer coefficient, etc., were obtained and analyzed. The secondary-side heat transfer coefficient reaches the highest value near the pseudo-critical point. The primary-side heat transfer coefficient increases with temperature because helium thermal conductivity increases with temperature. In the low temperature section with T22 as the tube material, the primary-side thermal resistance occupies more than 65%. In the high temperature section with 800H as the tube material, the thermal resistance of the primaryside and the tube wall are close. The thermal resistance of the secondaryside is always small. The secondaryside velocity varies intensely because of the severe change of density, while the primary-side velocity varies moderately. The hydraulic characteristic curves (pressure drop vs. flow rate) of different working conditions were calculated, and they all contain negative slopes which is not good for flow distribution and instability. The inlet throttling resistance coefficient of heat transfer tubes were added to avoid negative slopes of the curves. The critical throttling resistance coefficients of 100%-30% load working conditions are between 850-1600, and the value increases with decreasing power ratio.