Abstract: Numerical simulation of the flow and heat transfer of supercritical pressure water in horizon-tal smooth tubes has been carried out.The diameter of the tube was Φ32 mm,and the length of the tube was 8000mm.Variation of heat transfer coefficients along the tube was obtained at pressures of 26MPa and 34MPa,respectively,and the evolution characteristics of the secondary flows in the tube were analyzed.The local parameters,such as the inner-wall temperature and the specific heat were adopted in the present study to reasonably explain the variation of the secondary flows of the supercritical pressure fluid in heated tubes.Acomparison of the natural convection to that of the force convection was made through dimensionless num-bers Gr/Re2 and Gr/Re2.7,and the characteristics of flow and heat transfer in horizontal tubes was well ex-plained.