The invention discloses a guiding method and device used for positioning a steel open caisson, and relates to the field of bridge engineering construction. The guiding method comprises the following steps that at least four support anchoring piles and at least four cruciform beams are made, and each cruciform beam is provided with accommodating cavities; all support anchoring piles are vertically knocked and positioned into a pier well hole of the steel open caisson in a plug-in manner, the top of each support anchoring pile is fixed with one cruciform beam, and the interior of each accommodating cavity is provided with a top bracing device; and the steel open caisson is assembled in the pier well hole. Water is injected into a separation chamber of the steel open caisson, when the steel open caisson is in a vertical state, water injection is stopped, each top bracing device supports the inner wall of the steel open caisson, water injection is continued, the steel open caisson goes down, and the top bracing devices are dynamically adjusted to make the steel open caisson maintain a vertical state. According to the invention, the steel open caisson can vertically go down, and can be positioned accurately, synchronous operation can be achieved, process transformation is reduced, construction interference is reduced, water area occupied by construction is reduced, influence on river navigation is reduced, and the guiding method and device used for positioning the steel open caisson are beneficial to construction operations in narrow river areas.