The invention discloses an underwater position sinking method for a steel open caisson. The method is as follows: a positioning frame and positioning mark steel are arranged according to the geographical position of the taken underwater deposit on the seabed or water bottom, a high-precision attitude instrument and/or ultrashort baseline positioning equipment is utilized to measure coordinates of the positioning frame, a sampling beacon of the ultrashort baseline positioning equipment is arranged on the top of the steel open caisson at the same time, a crane ship is used to hoist the steel open caisson to cover at the periphery of the deposit, and finally a method of excavating sludge at the outer wall of the steel open caisson is used to help the steel open caisson continue to sink to the target depth. When the underwater position sinking method for the steel open caisson is adopted, a mode of excavating inside the steel open caisson is not adopted to auxiliarily sink and a method of assisting the steel open caisson to sink by a vibration hammer on the water surface is also not adopted, so the method can assure the accuracy and stability of the sinking of the steel open caisson, can also ensure that the safety of the deposit in the steel open caisson is not subject to the new damage during the sinking of the steel open caisson, short construction period at the same time, and greatly reduce the construction hazard brought by the ocean environment during the offshore operation.