Subcooled flow boiling is a very efficient heat transfer approach and can achieve a high heat transfer coefficient, so it is widely applied in thermal power, petrochemical, aerospace, and other engineering fields. In the present study, experiments are carried out to investigate flow boiling heat transfer characteristics in a vertical rectangular channel under high-pressure conditions of 0.5-0.75MPa. The cross-section of the channel is 10mm×10mm and the working fluid is deionized water. One side of the channel is heated and another three sides are designed transparently to record the boiling flow patterns. Five thermocouples are arranged on the back of the heating plate to monitor the wall temperature at different positions. The effects of pressure, inlet flow rate, heat flux and inlet temperature are investigated for further understanding of the heat transfer mechanisms. The visualized test results show the bubble nucleation, bubbly flow including isolated bubble and confined bubble, slug flow and pattern transition successively under different heating power by using a high-speed video camera. The post-processing of the time-averaged void fraction along the flow direction is analyzed and it's observed that the void fraction is similar at the same distance from the heating wall and it's reduced with the increasing distance from the heating wall.