Abstract: Water in the steam generator secondary circuit in the 250 MWt pebble-bed module high temperature gas-cooled reactor (HTR-PM) which was built by the Institute of Nuclear and New Energy Technology (INET) of Tsinghua University, is subcritical. Its pressure and temperature can increase to supercritical as the reactor provides high temperature helium up to 750 ℃, then the reactors and the supercritical steam generators can work with the supercritical steam turbine by adapting multiple-reactors with one-turbine unit design concept. Therefore, the flow, heat transfer and the flow instability of supercritical water in tubes are of great importance. RNG k-ε turbulence model coupled with enhanced wall treatment was applied in the simulation, and the numerical calculation results agree well with the experimental results of Yamagata, et al. The heat transfer coefficients of supercritical water in a tube under various heat fluxes were analyzed based on this model. The flow instability was also studied by linearly increasing the heating power with time. The results show that the inlet and outlet flow rates oscillate intensely once the flow enters the unstable zone and even reserve flow can be observed. Such phenomena should be avoided.