Modelling of the subcooled flow boiling is important in many industrial fields, such as in nuclear steam generators, satellites, and space vehicles. It is especially important in the nuclear engineering since an accurate knowledge of interfacial behaviors and void fraction distributions in reactor cores is necessary for various safety analyses. Currently, there exist lots of one-dimensional semi-empirical correlations in predicting the void fraction. However, most of the current correlations are based on experimental investigations which only considered limited ranges of working conditions. In addition, the mechanistic analysis for this process is still lacking. Therefore, the present paper aims to propose a new method for predicting the void fraction in a one-side heated annular channel in the subcooled flow boiling. Based on the existing bubble-layer-based model, the flow field is divided into two regions, which are the bubble layer region and the core region, based on the existence of a bubble layer adjacent to the wall. Then, bubble behaviors in each region, mass and energy exchanges at the interfaces of different regions, and variations of parameters along the channel direction are considered and analyzed through a new set of two-dimensional steady-state conservation equations of mass and energy. As a result, the present model shows high accuracy in the prediction of the void fraction and the verification is conducted by comparing the predicted values with corresponding experimental results.