This invention is one kind of hypothesized path boundary the auxiliary driving installment, is supposes in an automobile, as soon as in this automobile is equipped with uses to control the steering wheel rotation to change the component auxiliary, but this hypothesized path boundary the auxiliary driving installment including a processor, a phantom picks up the mold train as well as a secondary control mold train, this phantom picks up the mold train is supposes on the automobile picks up front the vehicles the phantom, and delivers gives the fore-mentioned processor analysis automobile whether to go the deviation traffic lane central committee, when the processor examines knows the automobile travel deviation traffic lane, namely the actuation states the secondary control mold train to rotate this auxiliary changes the component to help the automobile travel turning lane central committee; Is by, when automobile travel deviation traffic lane this invention may assist the driver to return the vehicles travel the traffic lane central committee, but makes the driver to have one kind of travel in a hypothesized concavity road surface the control feeling.