A variable compression ratio-applied control system using an ADAS and a control method thereof are provided. The system includes a driver assist unit for measuring the distance and the relative speed with a preceding vehicle, and a compression ratio changing unit for changing a compression ratio during traveling, and a neutral control mode for blocking power transfer by a transmission during traveling is implemented, a compression ratio changing unit is operated by using the information acquired through the driver assist unit in a state where the neutral control mode has been implemented, and the compression ratio is changed during inertia traveling during which the neutral control mode has been implemented.