A method and a HAC system controls a hill-state assist control (HAC) operation when restarting an engine of the vehicle having an idle stop and go (ISG) system for an automatic transmission. The method includes starting restarting of the engine by the ISG system and an HAC operation, monitoring the RPM of the engine, and releasing the HAC operation in a case where the RPM of the engine meets an HAC release condition, and maintaining the HAC operation until the HAC release condition is met in a case where the RPM of the engine does not meet the HAC release condition.