Abstract: Helical coiled steam generators (HCSGs) have been widely used in the small water cooled reactors, the high-temperature gas cooled reactors, and the liquid metal cooled reactors due to the characteristics of high compactness, rapid system response and high outlet vapor quality. During operation, the performance of HCSG degrades due to the effects from fouling deposition, chemical corrosion and thermal fatigue, which requires three-dimensional localized thermal-hydraulics parameters to estimate and quantify. However, most of the available studies on the thermohydraulics in HCSG focus on the one-dimensional modeling and the three-dimensional analysis on the slice of tube bundle. The study on the three-dimensional flow and heat transfer characteristics for the whole HCSG, which are required by the HCSG degradation analysis, is rare. Therefore, a numerical methodology to predict the fully three-dimensional thermohydraulics in the HCSG with consideration of primary-to-secondary side heat transfer was presented in this paper. The three-dimensional flow and heat transfer in the complex flow channel in the shell side of helical tube bundle were predicted by the computational fluid dynamics model with consideration on the effects of Prandtl number on turbulent heat transfer. The two-phase flow and boiling heat transfer in the helical tubes as well as the heat conduction cross the tube wall were modeled by a one-dimensional code with parallel multi-channel model. The coupling heat transfer was calculated at the outer surface of helical tube to consider the primary-to-shell side heat transfer, where the fine-to-coarse mesh mapping technique was employed to deal with the heat transfer between cells with different scales and the numerical convergence, respectively. The flow and heat transfer models for primary and secondary sides were validated separately against DNS data and experiment results. A prototype of lead cooled HCSG was employed to demonstrate the innovative analysis method. The three-dimensional flow and temperature fields in the shell side and one-dimensional thermohydraulics parameters inside each set of helical tubes can be obtained. The velocity among different sets of helical tubes is nonuniform, proving the necessity of three-dimensional analysis on the shell side flow field. Locations with local minimum velocity magnitude can be found, where the fouling deposition should be concerned. Furthermore, significant difference in wall temperature at the upwind and downwind sides of each tube can be observed due to the variations of local velocity along the circumferential direction. The large wall temperature difference would lead to the thermal fatigue of tubes. The results show that the current numerical method can predict the three-dimensional thermal-hydraulics parameters in the HCSG, which provides support for the structure optimization and performance degradation analysis of HCSG.