Abstract: Scaling analysis plays a key role in the design of passive test facilities. To deeply understand the mechanism of transient natural circulation in scaling process, the transient natural circulation based on a simplified reactor primary loop system was investigated with experiments and RELAP5 code. Scaling analysis was performed by means of H2TS method and DSS method, and the dynamic changes on key parameters under powerdown cases and powerup cases were analyzed. The results show that numerical results simulated with RELAP5 code are basically consistent with those experimental ones. Step changes below 5% of the initial power will not cause large flow fluctuation. The calculated results based on the two scaling methods can basically reflect the variation of prototype parameters. Under all cases, the corresponding mass flow rate and temperature difference between the cold and hot section fluctuate greatly within the initial stage of 0.5 cycles, and then behave relatively stable.