Abstract: There are a series of complex physical phenomena in once-through steam generators (OTSG), such as full-regime flow boiling and phase-change heat transfer from supercooled water to superheated steam, coupled heat transfer between tube side and shell side. At the same time, the structure of OTSG is also very complex. There are a large number of slender heat exchange tubes in the OTSG, and the difference between the radial size and the axial size is very large, that is, the size span is large. These problems and characteristics lead to the problems of high computational difficulty, low solution efficiency and poor solution stability when using computational fluid dynamics (CFD) and threedimensional refined modeling methods to analyze the three-dimensional temperature field of OTSG with large-scale heat exchange tube bundles. Based on the full-regime flow boiling, phase-change heat transfer model and the criterion model of boiling flow regime, a one-dimensional full-regime flow boiling and phase-change heat transfer analytical model were stablished in this paper. With appropriate parameter transfer, the cross-scale coupling solution between the one-dimensional boiling heat transfer analytical model on the tube side and the three-dimensional refined CFD model on the shell side was realized. This crossscale coupling calculation method can realize the independent coupling between each heat exchange tube on the water side and the threedimensional fluid domain on the sodium side. The proposed crossscale coupling numerical model was applied to the oncethrough steam generator of the Indian fast reactor to realize the independent coupling between each OTSG heat exchange tube and the shell side threedimensional sodium fluid domain, and the plugged tube condition analyses were carried out. The effectiveness of the proposed cross-scale coupling model was verified by comparison with experimental data. The study results show that the established crossscale coupling numerical analysis model between tube side and shell side can accurately predict the temperature distribution and the main heat transfer characteristic points of each heat exchange tube over the entire height. The calculation results can also give the temperature of different heat exchange tubes and the temperature difference between the tube walls of each heat exchange tube in detail. This method is also very convenient to realize the analysis of plugging conditions. The crossscale coupling solution method avoids the direct solution of complex threedimensional multiphase flow equations, and greatly improves the efficiency and stability of the numerical solution of the OTSG threedimensional temperature field. This study provides an efficient threedimensional refined analysis method for the design and safety verification of OTSG.