Abstract: The concept of the inherent safety characteristic on thermal-hydraulic of the pool-type sodium-cooled fast reactor was proposed and its formation mechanism was analyzed. That is, the excellent physical properties of sodium (high thermal conductivity, large difference value between melting point and boiling point at atmospheric pressure, and good fluidity) combined with the reasonable design of pool-type reactor (large sodium capacity, height arrangements between heat exchangers as well as cold sodium pool and core), make the reactor has a characteristic: In the early stage of reactor scrams, the sodium and metal components in the reactor vessel can be used as the effective heat sink of the core decay heat, and the heat generated in core can be removed into the sodium pool in time by heat conduction and natural circulation flow to ensure the core safety. Based on the actual reactor natural circulation test data of BN-600 and Phenix reactors, the natural circulation experiment data in the test device with sodium as working fluid and the natural circulation condition predictive calculation of the sodium-cooled fast reactors with typical layout using the different codes, the characteristic and its formation mechanism were demonstrated. The core decay heat can be transferred to the pool in time under the natural circulation condition because the good thermal conductivity of sodium and the natural circulation flow due to the reasonable reactor design result in the establishment of a good heat exchange relationship among assemblies by the wrapper walls and the sodium in inter-wrappers, then, on the one hand, the heat of the assemblies is discharged by the combined action of the sodium in the wrappers and inter-wrappers, on the other hand, the outlet temperature differences among assemblies are leveled to a certain extent, then the peak temperature is lowered. Based on the above research, for the pool-type sodium-cooled fast reactor, in the early stage of reactor emergency shutdown, the reactor’s inherent thermal-hydraulic safety characteristic alone can ensure the core safety without other heat sink, and in the later stage, it is only necessary to establish an effective heat sink from outside the reactor before the sodium temperature in the reactor rises to the safety limit. The effective heat sink can be the specially arranged decay heat removal system or the reactor vessel conventional heat loss item etc. And for the typical decay heat removal system setting heat exchangers in cold sodium pool or hot sodium pool, the system cannot play a role to removal core heat in the early stage of operation, at this time it must to rely on the inherent thermal-hydraulic safety characteristic as heat sink to ensure core safety.