The experimental investigation was performed in the full scale thermo-hydraulic simulation test loop (HRTL-5) for the 5 MW-Heating Reactor at Institute of Nuclear Energy Technology, Tsinghua University. Under different pressures the flashing in riser of natural circulation system and its effect on flow stability were studied by the method of visual observation in synchronism with data recording. The results show that the lower the system pressure, the more obvious the flashing in the riser, and the stronger the effect on flow stability in the natural circulation system. Under very low pressure even if subcooling at the exit of heated test section is relatively higher, the flashing riser will appears due to gradual energy accumulation by strong thermodynamic non-equilibrium process. Consequently, a long period flow oscillation is stimulated in the system, and the time of period can be up to 40 s. When the system pressure reaches to 1.5 MPa, the flashing in riser is vanished. Based on these experiments, the mechanism of flashing in riser and the flow instability stimulated by the flashing are basically explained.