Abstract: Under natural circulation condition, the non-uniform flow will appear in the parallel flow in the tubes of inverted U-tube steam generators. And reverse flow occurs within some tubes, which can greatly influence the characteristics of natural circulation. To study the characteristics of non-uniform flow, U-tubes were classified according to their lengths. By using the best estimate code RELAP5/MOD3.3, the transition processes of natural circulation with different coolant pump moments of inertia were studied. The results show that the increase of moment of inertia can postpone the occurrence of reverse flow, but can not change the distribution and flow rate of reverse flow.