The invention discloses a method for reducing the thickness of subsealing concrete of a steel cofferdam and preventing seepage, comprising the following steps of: a. assembling the steel cofferdam and welding an upper circle of shearing resistant structure and a lower circle of shearing resistant structure along the inner wall of the steel cofferdam; b. feeding the steel cofferdam in water; c. welding an upper circle of shearing resistant structure and a lower circle of shearing resistant structure on the outer wall of each steel protecting tube and sequentially punching the steel protecting tubes into a foundation in the steel cofferdam; d. constructing all drilling piles; e. cleaning the foundation in the steel cofferdam and flushing the steel cofferdam and the shearing resistant structures underwater on the steel protecting tubes; f. pouring the subsealing concrete to be combined with the steel protecting tubes, the steel cofferdam and the shearing resistant structure into a whole;and g. pumping water from the steel cofferdam and constructing a bearing platform. According to the method, by arranging the shearing resistant structures, after water in the steel cofferdam is pumped, the force transfer mode of the subsealing concrete and the inner wall of the steel cofferdam and the outer walls of the steel protecting tubes is converted into a bonding force and a shearing forcefrom the mutual bonding force, so that the dereference of the concrete bearing capacity accords with the reliable basis and the thickness of the subsealing concrete can be reduced; and the water seepagae in the steel cofferdam can be prevented.