Abstract: Thermal-hydraulic phenomenon of coolant in the primary circuit of pool type fast reactor under accident decay heat removal conditions is complicated because the layout of direct heat exchanger. In order to further understand the behavior natural circulation of coolant in primary circuit system of pool type fast reactor under accident decay heat removal conditions, a visual slab water model was established to simulate main component in primary circuit system and natural circulation experiments had been carried out on this model. Temperature response and flow field of coolant in core, pool and inlet and outlet of each heat exchanger were obtained. According to the transient temperature at the inlet and outlet of core assembly, in the initial stage of establish of natural circulation, the temperature at outlet remained relatively stable for a short period of time without increasing. After the temperature of cold pool rise, the temperature at outlet of core started to increase. So the cold pool has a significant impact on the temperature rise rate of short term natural circulation. In this experiment, the natural circulation flow in the intermediate heat exchanger and inter-wrapper space of core was observed. The higher temperature fluid from outlet of core floats directly to the top of hot pool and lower temperature fluid in the hot pool sinks, which forming a local circulation flow in hot pool. Due to the intermediate heat exchanger loses its ability of heat transfer, the temperature of cold pool becomes higher and higher, with the development of natural circulation, and finally, temperatures of inlet and outlet of intermediate heat exchanger and cold pool are relatively close when the flow reaches to a stable state. Under influence of hot water form core and cold water from outlet of direct heat exchanger, a thermal stratification is formed and the position of interface of hot water and cold water is near top of the core. In this experiment, mixing of hot water and cold water is observed in the top of core, which forms significant temperature fluctuations, even in the place 20 mm below the outlet in the core assembly, this fluctuation still exists, indicating that the mixing effect can penetrate into the interior of the core assembly, which may cause a disadvantage to the establishing and maintaining of natural circulation through the core. All this result can enable us to fully understand the mechanism of natural circulation and provide support for the improvement of models in system analysis code.