The invention provides a steel cofferdam for repairing a channel. The steel cofferdam is a three-sided water retaining cofferdam enclosed by a longitudinal cofferdam and two transverse cofferdams, wherein the longitudinal cofferdam is spliced by two longitudinal joint-section cofferdams and one or more longitudinal middle-section cofferdams; each transverse cofferdam is spliced by a transverse joint-section cofferdam, one or more transverse middle-section cofferdams and a transverse tail-section cofferdam; each section of cofferdam is composed of a floating box type bottom plate and a sealing water retaining plate perpendicular to the floating box type bottom plate; each floating box type bottom plate is provided with connecting members which are spliced with an adjacent floating box type bottom plate; and each transverse cofferdam is inserted into a groove formed in the corresponding longitudinal joint-section cofferdam through a connecting insertion block arranged on the transverse joint-section cofferdam of the transverse cofferdam and is in sealing connection with the longitudinal cofferdam. The steel cofferdam is of a simple structure, is convenient to use, low in cost and good in repair effect and can repair the channel under the environment without water.