Abstract: FLUENT software is utilized to numerically simulate internal three-dimensional flow field of all-welded ball valve (cylindrical) and the simulation is used to explore the flow characteristics of the valve from closing to opening fully. Taken incompressible medium as an example, ball valve's internal flow field changing situation was analyzed under 7 different open degrees (10%, 30%, 50%, 70%, 90%, 100%), and the valve inlet and outlet pressure and flow rate were obtained. Then the flow resistance coefficient was worked out and the water flow coefficient under different opening degree was calculated. By compared the properties of compressible and incompressible medium and using density ratio conversion, the flow coefficient of compressible natural gas under different open degrees was computed and the relationship between compressible gas flow coefficient and opening degree was obtained. Simulation results were in accordance with realities. The research has guiding significance for the study that the flow state of compressible medium in ball valve.