The in-cylinder tumble intensity of GDI engine is crucial to combustion stability and thermal efficiency, required to be different for the different operation conditions. A new variable tumble system (VTS) applied to GDI engine was introduced to meet tumble ratio requirements in various situations. The transient gas exchange of four combustion systems all were investigated during both intake and compression strokes based on CFD simulation, namely (1) Case 1-Intake port B (with flap valve)/Spherical piston crown; (2) Case 2-Intake port B (without flap valve)/Spherical piston crown; (3) Case 3- Intake port A/Spherical piston crown; (4) Case 4-Intake port A/Dented piston crown. The simulated results of dynamic tumble ratio showed that during the whole intake process the dynamic tumble ratio of Case1 was obviously higher than other Cases with the same boundary conditions, and the maximum value was about 5∼6 times higher. The crank angle range, in which the strong tumble motion fully developed, was merely about 70°CA, roughly from 410°CA to 480°CA. Given that the in-cylinder airflow could not form rotation motion around one axis that parallel to Y vector in this crank angle interval, with the intake valve opening gradually becoming smaller, it was impossible to generate higher tumble intensity again in the next crank angles. Moreover, for Case1 with a flap valve in intake manifold, the mean velocity and turbulence kinetic energy all were almost twice than those of other Cases when piston close to TDC. In terms of airflow streamlines or contours of in-cylinder flow field it was concluded that the geometric shape of intake port without flap valve should be modified to further increase tumble intensity, specially, in some locations that sensitive to tumble ratio. While the piston crown shape is not vital to tumble intensity change in terms of calculated results. Eventually for the simulated cases the corresponding prototypes of cylinder head were manufactured and processed to carry out a series of steady state flow testing. For tumble ratio there was significant difference among all Cases, which consistent with simulated results. The tumble ratio of VRS will mostly be changed from 1.56 to 2.8, as flap valve fully closed or open, respectively.