Abstract: The helical-coiled once-through steam generator (H-OTSG) is widely used in small modular reactors due to its compactness and higher heat transfer efficiency. In this study, an experimental and numerical simulation study of the thermalhydraulics characteristics of HOTSG under different thermal power conditions was carried out, and the experimental results were used to verify the onedimensional system code SGTH1D. The configuration consists of 85 helically coiled tubes divided into ten layers according to the coil diameter. Both steadystate experiments and flow instability experiments were conducted under thermal power condition ranging from 0.6 MW to 2.3 MW. Firstly, the steadystate experimental results show the difference between the shellside inlet temperature and the tubeside outlet temperature increases with thermal power, which is due to the decreasing of the heat transfer capacity of the HOTSG. Sensitivity analysis of the system parameters shows that the shellside pressure has little effect on the average heat transfer coefficient. And the average heat transfer coefficient also is insensitive to tubeside pressure and shellside inlet temperature under low thermal power conditions. However, under low thermal power conditions, the average heat transfer coefficient increases with the decrease of the tubeside pressure and the increases of the shellside inlet temperature. Secondly, the flow instability experimental results show that the increase of the thermal power can stabilize the HOTSG and decrease the inlet throttling of the flow instability thresholds because the length of the subcooled singlephase region increases with the thermal power. The sensitivity analysis of the system parameters shows that the increase of the tubeside pressure and the decrease of the shellside inlet temperature are beneficial to the stability of the system. The influence of the shellside parameters on the flow instability threshold is weaker than that of the tubeside parameters. Finally, a numerical study on the thermal-hydraulics characteristics of the configuration was carried out in the present study. By comparing the experimental data with the numerical simulation results of the SGTH1D code, the accuracy of the SGTH1D code in predicting the steadystate heat transfer characteristics and flow instability of configuration was verified. The SGTH1D code can accurately predict the heat transfer rate of the HOTSG under various thermal power conditions, and the prediction errors of shellside and tubeside outlet temperatures are within ±1 ℃. The numerical results of flow instability characteristics of the SGTH1D code are conservative at low thermal power condition, and the error of numerical results is within ±20% when the thermal power is larger than 1.2 MW.