Abstract: In order to analyze the influence of different thermal parameters on the characteristics of heat transfer and flowing during the transition from forced circulation to natural circulation and during the stable natural circulation, the experimental study was carried out in a vertical test section with 6 mm inner diameter. The experiment was carried out under different thermal-hydraulic conditions, including different inlet subcoolings, heat fluxes and initial mass fluxes. The experimental results show that the inlet subcooling and heat flux have great effect on the characteristics of flowing and the heat transfer during the transition from forced circulation to natural circulation, and also during the stable natural circulation period. However, the effect of the initial mass flux is not significant.