The invention discloses a construction method of a marine self-floating type steel soleplate single-wall suspension box cofferdam and a single-wall suspension box cofferdam, wherein the construction method comprises the following steps of: firstly installing a soleplate suspended shoulder pole girder and lowered guiding device of the cofferdam, then installing a steel soleplate, a lateral plate and an inner support, then carrying out the construction of lowering, fixing and bottom sealing by concrete of the cofferdam and finally carrying out the construction of a bearing platform. The cofferdam can be repeatedly used after being dismantled. The marine self-floating type steel soleplate single-wall suspension box cofferdam comprises the steel soleplate, the lateral plate, the inner supportand the soleplate suspended shoulder pole girder and lowered guiding device. The steel soleplate of the invention is of a self-floating type assembled structure, an inner inclined strut of the cofferdam is utilized as a distributing girder suspended on the cofferdam when in the construction of bottom sealing by concrete, the lateral plate adopts a massive integral design, and meanwhile, an inner guide girder and the lateral plate are combined into a whole, thereby improving the integral rigidity of the lateral plate and also reducing the assembled working procedures of the cofferdam. The invention has the advantages of effective acceleration of construction schedule, simple and convenient construction operation, high repeated use ratio, safety, reliability and less cost investment.