Abstract: An experimental investigation of flow instability in a natural circulation system with long and straight riser was carried out. Meanwhile, a series of typical flow instabilities under low pressure and high inlet subcooling conditions were analyzed in detail. The experiment results show that natural circulation characteristics and flow instability type are affected by the structure and thermal boundary of fluid. Under high inlet subcooling condition, the system could hardly return to two-phase stable state if heat flux exceeds a critical value. With the increase of heat flux, geysering flow instability and compound dynamic instability occur in natural circulation system. According to the experiment data, instability boundary map under high subcooling condition is obtained. Natural circulation driving pressure head and resistance pressure head are strongly affected by the thermal-hydraulic state in heated channel and adiabatic riser. The accumulation of massive bubbles which triggers periodic local high pressure is the primary reason of large oscillation and flow reversal.