Abstract: Integral thermal hydraulics test plays a key role in the safety verification of PWR. The scaling analysis of the circulation characteristics in the primary loop provides the theoretical basis for the integral test facility design. Based on the two-phase drift flow model, the governing equations of forced circulation and natural circulation were established. The initial conditions were applied to nondimensionalize the governing equations resulting in the similarity criterions of transient from the forced circulation to the natural circulation for the integral test facility. The experimental method which can simulate the prototype main pump coast-down and satisfy the similarity requirements of the transient between circulations was proposed.