Abstract: The best estimate system analysis code RELAP5 is used to the modeling and numerical calculation of a small reactor, and on this basis, the natural circulation flow stability analysis for small reactor is carried out. It is concluded that the core power and moderator reactivity feedback have influence on the natural circulation flow stability of the primary loop of small reactor. The system has high stability when operating at high power. Under the reaction feedback of moderator with larger absolute value, the flow transient response appears overshoot. The reactor trip signal of the core outlet（ascending section） low subcooling can protect the small reactor within the design range from appearing natural circulation flow instability.