Abstract: The natural circulation capacity is the important parameter that characterizes the safety capability of a nuclear reactor. In order to analysis the passive safety of a pool-type research reactor, and determine the extreme levels of heat-carrying capacity under natural circulation condition, according to the special core structure, a method was proposed to carry out natural circulation experiment, and a software was developed to analysis the natural circulation capability. The calculation results of core flow satisfy experimental values well, and the relative deviation is less than 1.6%. Under the condition the pool water temperature is 40 ℃, the natural circulation capacity of the reactor is no less than 710 kW according to the calculation result of the code. It indicates that this pool reactor has good passive safety ability.