The invention relates to a method for threading and installing a holding beam at the bottom of an underwater steel open caisson, comprising the steps of: installing a holding beam threading rack on one side of the steel open caisson, connecting a hauling wire rope on a lifting jack of the threading rack, threading the holding beam from an mounting hole arranged on the other side of the steel open caisson and connecting on the hauling wire rope, pulling the hauling wire rope step by step by using the pulling force of the lifting jack until pulling the holding beam to the side provided with the lifting jack, protruding two mounting holes positioned at the crotch of the holding beam and passing through the steel open caisson out of the steel open caisson, and installing a pin on the crotch to allow the a closing plate at the other end of the holding beam to be tightly attached to and fixed on the wall of the steel open caisson. The holding beam is provided thereon with a high pressure water pipe, and the displacement during the pulling process of the holding beam is controlled by using high pressure sprinkling. By using the method, the steel open caisson can seal the bottom after accommodating the salvaged matters, a plurality of holding beams and the steel open caisson can form a commonly forced container, so as to reach the purpose of completely moving the salvaged matters.