Abstract: In order to investigate the principle of working in a diesel variable swirl system,the flow performance of a set of inlet ports with a swirl control valve was studied on a steady flow rig,and PIV was made use of for taking photographs of the flow fields near the inlets,through which the mass flow rate of the ports was counted.At the same time,the testing system was analyzed by CFD method with the simulation code AVL-FIRE.Experiments showed that the mass flow rate and the air velocity of diesel inlet ports can be changed by the swirl control valve.The swirl in the cylinder can also be changed by the swirl control valve.