The experiment was carried out on fhe test loop HRTL-5, which simulates the geometry and system design of a 5 MW nuclear heating reactor The analysiswasbased on a one-dimensfonal two-phase flow drift model with conservation equations for mass, steam, energy and mo-mentum. Clausius-Clapeyron equation was used for the calculation offlashing front in the riser. A set ofordinary eguations, which describes the behavior oftwo phase flow in the natural circulation system. was derived through Integrafion of the above conservation equations for the subcooled boiling region, bulk boiling region in the heated section and for the riser. The method of time-domain was used for the talc uIation Both static and dynamic results are presented System pressure, inlet subcooling and heat flux are varied as input parameters. The results show that subcooled boiling in the heated section and void flashing In the riser have significant jnfluence oN the distribution of the void fraction, mass flow rate and flow instability ofthe system, especially at low pressure The response of nlass flow rate. after a small disturbance in the heat flux is shown, and based on it the instability map ofthe system is given through experiment and calculation. There exists three regions in the instability map of the investigated natural circulation system, namely, the stable two-phase flow region, the unstable bulk and subcooled boiling flow region and the stable subcooled boiling and single phase flow region. The mechanism of two-phase flow osciHation is interpreted.