In order to reduce the aerodynamic drag, a domestic car was selected as study object to research the size and the layout of the vortex generator with the method numerical stimulation. By comparing the values of drag coefficient (Cd) the cars with vortex generators (VGs) and the original and then analyzing the mechanism of drag reduction, a final optimized strategy was achieved. It was concluded that VGs can lower the Cd of a car but the effect depends on the size and the layout. Our research achievements can serve as reference for automobile aerodynamic drag reduction, and are also significant to energy saving and emission reduction.