Abstract: Within the pressure range from 11 to 19 MPa, mass velocity from 700 to 1300 kg/（m²·s）, and inner wall heat flux from 200 to 1000 k W/m², experiments were performed to investigate the heat transfer characteristics of water flowing in subchannel with square distribution in SCWR. The subchannel was formed according to the fuel rods of the diameter 8 mm, and the pitch-to-diameter ratio was 1.2. The results show that the heat transfer characteristics of subcritical pressure water flowing in the subchannel with square distribution are significantly affected by the inner wall heat flux. Heat transfer deterioration more likely happens with higher inner wall heat flux. Heat transfer deterioration is more likely happened under low mass velocity, while it has little influence on the heat transfer characteristics at high mass velocity. The pressure has significant effects on the heat transfer characteristics. The higher pressure, the more likely the heat transfer deterioration occurred. The lower critical steam quality and the widely enthalpy region where heat transfer deterioration happen.