Abstract: It is of great significance to the safety design and analysis of reactor to deeply understand the influence mechanism of the rod bowing in the phase distribution and the evolution of flow characteristics. In this paper, four rod bowing sub-channels with different gap closures（0%, 50%, 80% and 100%） were modeled. Based on the VOF model, the air-water two-phase flow in sub-channels was calculated using unsteady-state and implicit-separation algorithm. The calculated condition contains the bubbly flow（inlet void fraction is 0.1） and the cap flow（inlet void fraction is 0.5）. In addition, the inlet mass flow rate of the mixture phase is 1.2 m/s. Based on the analysis of average void fraction at the cross section, local bubble evolution and local secondary flow, the effect of rod bowing on the air-water two-phase flow was studied. The results indicate that the rod bowing has little effect on the average void fraction at the cross section, but a significant influence on the size and distribution of local bubbles. This causes that the curved and diagonal wall surfaces in the sub-channel are prone to be exposed and consequently occurrence of CHF.