The invention discloses a controlling method a for vehicle electric power steering system (EPS). The method comprises the steps that the priority levels of all external requests of EPS are customizedby writing in configuration codes; when EPS receives two or more external requests, priority level information of the external requests is automatically obtained by the EPS according to the written configuration codes; the validity of the external requests are judged according to the priority levels; the external requests are sequentially executed according to the prior levels according to a judging result.