A variable intake cam was applied on a turbocharged gasoline direct injection engine to study its effects on engine fuel economy at partial load. The cam was composed of a long travel cam and short travel cam, the switch between them was realized by an ECU controlled electromagnetic valve. The results show that short travel cam can realize miller cycle through early intake valve closing (EIVC) with the reduction of pumping loss and incylinder residual burned gas due to higher intake pressure. Applying short travel cam can reduce brake specific fuel consumption by 1.9%-3.4% at different loads based on different mechanisms of fuel economy. For restricted operating conditions of non-knock such as the average effective pressure of 0.5 MPa, short travel cam will prolong combustion duration and the main reason of fuel consumption improvement are the reduction of unburned loss and mechanical loss. For restricted operating conditions of knock such as the average effective pressure of 0.8-1.7 MPa, short travel cam will improve combustion phase obviously, and the fuel consumption improvement is due to improved indicated thermal efficiency led by the increase of combustion constant volume.