Abstract: In the process of oil-water annular flow pipeline transportation, the stability of oil-water annular flow is more affected by the change of flow-path section the moment a ball valve is opened or closed. In this paper, the Volume of Fluid (VOF) model and Continuum Surface Force(CSF) model were used to track the oil-water interface. Then, a numerical simulation control equation was established by using standard k-ε turbulent model. Finally, the flowing state of oil-water annular flow inside the ball valve was analyzed under different inflow rates and different openings, and an oil-water annular flow test platform was established for verification. It is shown that numerical simulation results are consistent with test results. Obviously, the flowing state of annular flow inside the ball valve can be analyzed well by means of this numerical simulation. Valve opening has effects on the flow pattern, the oil-phase volume fraction, the pressure difference between two ends of a valve and the wall sticking distance of oil phase. When the valve opening is small, the oil-water annular flow inside the valve and in the outlet pipeline behind the valve is damaged, oil-phase volume fraction is smaller, the pressure difference between two ends of the valve is larger, and oil phase is easily stuck to the pipe wall. Above mentioned parameters are also affected by the inflow rate. Higher inflow rate can improve the stability of oil-water annular flow.