In order to study the characteristics of heat transfer in subcooled flow boiling of circulating cooling system, the unsteady numerical calculation of the subcooled flow boiling process of a horizontal pipe was carried out based on the VOF multiphase model and the Lee phase change model. The correlation of boiling onset proposed by Bergles was introduced for correcting the Lee model, taking into account the influence of the boiling onset. The heat transfer properties of the subcooled flow boiling process in the horizontal pipe and their variation rules were examined from two aspects: the thermal boundary layer development and the boiling development. The distribution regularity of relevant parameters under different heat flux conditions was summarized, as well as the distribution rules of heat transfer coefficient along the flow direction. Results showed that the development of the thermal boundary layer and boiling would enhance the instability of the flow field, and the fluctuation amplitude of the heat transfer coefficient in the rear was twice as large as in the front. Moreover, the increase of the heat flux accelerated the variation of the flow and heat transfer parameters along the flow direction. In the 250 kW/m² condition, the area affected by the thermal boundary development was about 60% that of the 150 kW/m² condition. The heat transfer coefficient in the front of the heating section showed a descending trend with the development of the thermal boundary layer. Therefore, when the heating area is in high flux condition, we can shorten the length of the heat exchanger to increase the heat exchange efficiency and weaken the fluctuation of the heat transfer coefficient caused by flow boiling.