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Subcooled flow boiling contains complex bubble behaviors and interactions. Accurate prediction and analysis of the subcooled flow boiling are of great significance for the safe operation and economy of the nuclear reactors. The bubble-layer-based model is a new theoretical model in predicting the flow boiling, which divides the flow filed into two regions in the radial direction, which are the bubble layer region and the core region. Bubble behaviors in each region, mass and energy exchanges between different regions, and variations of parameters in the axial direction are analyzed and reflected through a set of two-dimensional steady-state conservation equations of mass, momentum and energy. Therefore, this model can provide more detailed information of the flow field than current one-dimensional models. However, this bubble-layer-based model for the subcooled flow boiling has only been verified in conventional tubes. When the channel type and sizes change, the accuracy of the model is still uncertain. Considering that rectangular channels are widely encountered in multiphase flow equipment, this paper aims to further extend this model to the prediction of the subcooled flow boiling in rectangular channels. Firstly, several flow boiling experiments in both conventional rectangular channels and narrow rectangular channels were selected. The working conditions are 0.1-8 MPa for pressure, 211-1 700 kW/m2 for heat flux, 240-2 200 kg/(m2·s) for mass flux, 4.42×103-3.72×105 for liquid Reynold number and 0.83-1.76 for Prandtl number. Then, variations of void fraction obtained by this model were compared with these experimental data. The results show that this bubble-layer-based model shows high accuracy when it is applied to both conventional rectangular channels and narrow rectangular channels, which reflects that the channel size and shapes have limited impact on the accuracy of this model. In addition, to further verify the accuracy of this model, one of the existing models (Cai model) in predicting the void fraction in subcooled flow boiling was also selected for comparison. The results show that the bubble-layer-based model shows higher accuracy than Cai model, with the relative error of 32.7% and 52.6% respectively. Finally, this model was applied to the analysis of the flow boiling in one narrow rectangular channel. It describes the variations of several two-phase flow parameters, such as the void fraction, pressure, velocity and temperature. These results not only provide a better understanding of the mechanism of subcooled flow boiling, but also provide basic data for the thermal physical coupling analysis of the rectangular fuel elements in nuclear reactors.