Miller cycle can improve the thermal efficiency and brake specific fuel consumption by changing the gas distribution strategy to make the expansion ratio greater than the actual compression ratio and is hence considered as one of the important ways to improve the fuel economy of small GDI engine. However, the microscopic mechanism about the effect of Miller cycle on the combustion performance of gasoline engine is not clear. So the effects of different Miller cycle strategies on the intake and in-cylinder combustion processes of GDI engine were studied based on numerical simulation by using 1D GT-POWER software and 3D CFD CONVERGE software. The results show that the early intake valve closing (EIVC) strategy is better than the late intake valve closing (LIVC) strategy in improving the engine fuel economy at low and medium speeds, without much negative impact on the torque. The LIVC strategy can significantly increase the in-cylinder turbulence energy during combustion and hence improve the fuel-gas mixing process. In addition, the overall in-cylinder temperature is lower and the hightemperature region is smaller for the LIVC strategy after ignition, so it is better than the EIVC strategy in suppressing engine knocking.