Abstract: Due to the compact structure, high heat transfer efficiency and strong reliability, helical coil once-through steam generator (HCSG) is widely used in advanced small reactors. In HCSG, the liquid in the tube side is heated by the helical coils so that subcooled water in the tube side is transferred into superheated steam, and the entire heat transfer regime is covered. Moreover, the evolution mechanism of thermalhydraulic behaviors is complex because it is affected by the helical geometry, the centrifugal force and the secondary flow. Hence, the development of advanced HCSG was limited due to the lack of more accurate models of heat transfer and regime transition. With the financial support of the three major nuclear power groups in China, experimental and theoretical research on heat and mass transfer behaviors in helical coils was carried out systematically. Different helical coils were investigated and comprehensive parametric analysis of heat transfer was conducted to provide a better understanding of different heat transfer regimes. According to our research, a fundamental database of heat transfer and flow resistance in helical coils with a wide range of geometric dimensions and thermalhydraulic parameters was established. In addition, the transition pattern of the circumferential nonuniform heat transfer mechanism was clarified. Due to the secondary flow, it consequently leaded to the nonuniform distribution of flow field. The heat transfer and flow resistance characteristics were notably affected by the special behaviors of liquid in the helical coils. Hence, based on mechanism analysis, several models were proposed to construct a high precision library of allregime prediction model, such as a threeregion dryout quality model, a postdryout heat transfer model based on dryness gradient and a widerange model for twophase frictional multiplier factor. For the model of dryout quality, it takes the relative intensity of gravity, buoyancy and centrifugal force into consideration. Since different dominant forces lead to different heat transfer behaviors, a piecewise correlation is established to give more accurate predictions. As for the postdryout heat transfer model, it considers the influence of dryness gradient and achieves a more accurate prediction. For the twophase pressure drop model, the modified frictional multiplier factor is adjusted according to our database. Then, a prediction model was proposed which was also validated by other researchers and had great prediction accuracy in a wide range. In all, this work includes part of our investigation on the heat transfer of helical coils and provides a highprecision prediction in the entire heat transfer regime, which meets the design requirement of HCSG for advanced reactors.