Abstract: Natural circulation flow excursion has been observed on the test loop HRTL-5 under low steam quality conditions,which simulates the geometry and system design of the 5MW full power natural circulation Nuclear Heating Reactor.By means of a Four-Equations-Drift-Flux Model,which has considered the sub-cooled boiling,condensation and void flashing,the computational codes have been developed,from which the natural circulation characteristic curves and its operation curve have been obtained,and the bifur-cation curve and the static instability boundary have been determined.Therefore the Method of Characteristic Curves Analysis has been presented.Analytical results show that the Characteristic Curves Analysis Method is the effective approach to study the natural circulation static flow instability;Decreasing the heat flux or outlet two-phase resistance,and increasing the system pressure or inlet single-phase resistance are useful to avoid the natural circulation flow instability.