Abstract: Within the range of pressure from 22.5 to 30 MPa,mass flux from 1009 to 1626 kg/（m²·s）,and inner wall heat flux from 216 to 822 kW/m²,an in-depth experiment was conducted under supercritical pressure to investigate the heat transfer characteristics of water in vertical upward smooth tube.The heat transfer characteristics of water under supercritical pressure were obtained in the experiment.The effects of pressure,inner wall heat flux and mass flux on the heat transfer coefficient and inner wall temperature were analyzed,the heat transfer mechanism was discussed,and the corresponding empirical correlations were also presented.The experimental results show that when the bulk fluid temperature is near the pseudo-critical temperature,the tube wall temperature increases slowly with the fluid enthalpy,the heat transfer coefficient gets larger abruptly and the heat transfer enhancement phenomenon occurs in the smooth tube.Otherwise,the tube wall temperature increases obviously with the increasing fluid enthalpy and the heat transfer coefficient is low.With the increase of pressure and inner wall heat flux,and with the decrease of mass flux,the inner wall temperature increases,the heat transfer coefficient decreases and the heat transfer enhancement is weakened.With the increase of inner wall heat flux,the maximum of heat transfer coefficient appears ahead.