Compressed Natural gas (CNG), as a large reservation and clean alternative fuel, has been widely used in the automotive industry recently, especially for heavy-duty commercial vehicles. However, the lower flame speed of natural gas leads to low thermal efficiency, even worse contributes to abnormal knock combustion at high load aggravating engine thermal load and reliability. In this study, an inner-convex piston is comparatively evaluated with the original piston of a heavy-duty natural-gas engine by three dimensional computational fluid dynamics (3D-CFD) simulations coupled with detailed chemical reaction. The numerical results show a high correlation between heat release rate and in-cylinder turbulence intensity and the new geometry piston can strengthen turbulence intensity obviously which promotes the heat release dramatically compared with the original piston. Afterwards the new piston was manufactured and a comparative experimental study was conducted on a single-cylinder engine which operated at 6 bar and 12 bar IMEP loads including lean-burn and stoichiometric combustion. The test results indicate a similar trend as the simulation consequences that the inner-convex piston accelerates the flame speed and shortens the combustion duration due to high turbulence level. Therefore, thermal efficiency and emission performance are improved, particularly more significant for high load under lean-burn.