Abstract: Under the condition of natural circulation, reverse flow problem may appear in some Utubes in the steam generator. To alleviate reverse flow, a preliminary design scheme of an asymmetric Utube was presented and the reverse flow characteristics of asymmetric Utube were analyzed under natural circulation condition by theoretical analysis and numerical simulation. The relationship between flow rate and pressure drop of asymmetric Utube was established for theoretical analysis. An asymmetric Utube calculation model and system analysis model were established for a certain type of nuclear power plant, and the operation characteristics of different optimization schemes were numerically simulated by RELAP5/MOD3.2 program. The results show that, by increasing the height difference between the descending and ascending sections of asymmetric Utubes, the number of Utubes in reverse flow decreases and the total natural circulation flow increases. Under the condition of the secondary side passive residual heat removal, the asymmetric U-tube has a more obvious mitigation effect on reverse flow.