Computational fluid dynamics is used to simulate the hydraulic efficiency of the ozone contactor in A water plant in Beijing and to optimize the contactor by calculating the retention time distribution (RTD) of the fluid. The result shows that the value of T10 /HRT of the existing contactor is 0.466, and the phenomenon of obvious short circuiting and backflow which result in low hydraulic efficiency exists in the contactor. After optimization, the value of T10 /HRT of the contactor in the presence of guide plates is 0.561, making the hydraulic efficiency increased by 20.38%. However, the value of T90 /HRT is 1.316, which means that the phenomenon of backflow existed. The value of T10 /HRT of the contactor in the presence of horizontal baffles is 0.540, making the hydraulic efficiency increased by 15.88%, and the value of T90 /HRT reduces to 0.821. That means the concentration ratio of RTD increased, and the hydraulic efficiency of whole flow field is improved significantly in the presence of horizontal baffles.