The invention discloses a method for closing the bottom of a steel boxed cofferdam and gaps among river beds. The method comprises the following steps: a, the steel boxed cofferdam is assembled on a shore side; b, retaining beams are mounted in areas needing to be closed and keep a certain distance from the outer wall of the cofferdam, and connecting plates are mounted between the retaining walls and the outer wall of the cofferdam; c, a plurality of movable inserting plates are mounted between the retaining beams and the outer wall of the cofferdam and hung on the outer wall of the cofferdam by using steel wire ropes, and adjacent movably inserting plates are clung to each other; d, the cofferdam is moved to designed positions of the river beds; and e, the steel wire ropes for hanging the movable inserting plates are untied, the movable inserting plates are lowered, and then the bottom of the cofferdam and the gaps among the river beds are closed. The method has the advantages that the plugging effect is good, the ability of adapting to water flow is strong, the plugging speed is high, the operation is convenient and the safety is high, and has not need to be operated by a diver through diving into the water bottom.