Abstract: Generally, the subcooled flow boiling can be divided into two parts along the channel direction, which are the highly subcooled flow boiling and the slightly subcooled flow boiling. There exist large differences in bubble behaviors and interactions in each part. Modelling of the subcooled flow boiling is very important in many industrial fields, such as in nuclear steam generators, satellites, and space vehicles. However, subcooled flow boiling is a very complex process which contains various bubble behaviors, such as sliding, liftoff, and condensation. Currently, the prediction of this process mainly relies on experiments, theoretical analysis and numerical simulations. With the development of the research on bubble dynamics, it becomes possible to analyze the heat transfer mechanisms at the micro level. Therefore, based on the bubble dynamics and the bubble boundary layer model, a theoretical model was developed for the prediction of the subcooled flow boiling which divided the flow field into different regions in the radial direction. According to the concept of the separated flow model, bubble behaviors in each region, mass and energy exchanges at the interfaces of different regions, and variations of parameters along the channel direction were considered and analyzed through a new set of two-dimensional steady-state conservation equations. Once the flow field in the bubble layer region was obtained, the ONB point and OSV point were also determined through this model. The present model was verified with experimental data of void fraction and liquid temperature with the mean absolute error equal to 22.3% and 0.3% respectively. The application ranges of the present model are 0.827-4.5 MPa for pressure, 520-1 440 kg/(m²·s) for mass flux, 243-888 kW/m² for heat flux, 6.1-15.4 mm for hydraulic diameter. This model was also applied to the prediction of subcooled flow boiling in the fuel element channel of nuclear reactors. The results show that the length of the whole subcooled boiling section accounts for 52.6% of the total length, and the positions of ONB point and OSV point are 1 764 mm and 1 300 mm away from the channel outlet respectively. Present model provides a better understanding of the mechanism of slightly subcooled flow boiling compared with existing one-dimensional empirical or theoretical models, since more detailed information including the radial mass and energy exchanges can be revealed. In another aspect, the newly developed model also provides a simple, fast and stable method for the predictions of subcooled flow boiling especially for industrial applications compared with the two-dimensional or three-dimensional numerical simulation method.