In order to study the flow instability in the tube during the deep peak shaving and low load state of supercritical units, the flow instability in the inclined smooth tube with 20° inclination angle was experimentally studied and analyzed. The typical oscillation characteristics of nucleate boiling starting point oscillation, pressure drop oscillation and density wave oscillation were summarized, and the effects of mass flow rate, inlet subcooling and upstream compressible volume on nucleate boiling strarting point oscillation and density wave oscillation under low pressure parameters were emphatically analyzed. The results show that the flow pulsation in the tube can be weaken and the flow stability of the working fluid in the heating tube can be improved due to the increase of mass flow rate. The influence of inlet subcooling and compressible volume on the oscillation is dual and the law is relatively complex. In addition, a typical one-dimensional single channel vertical tube instability calculation loop was established, and the variation of heat flux and mass flow were calculated and analyzed. According to the results, the inlet and outlet flows oscillate inversely with time, and the critical heat flux and critical mass flow rate of loop instability are obtained. In the low load operation condition of the unit, the heat flux of the heating tube is much lower than the critical heat flux, and the mass flow rate is much higher than the critical mass flow rate, so as to ensure the stability of the working fluid flow in the heating tube. The flow instability of the working fluid in the inclined tube and the vertical upward heating tube can be determined by dimensionless parameters.