The invention discloses a combined cofferdam for a sludge river bed and a construction method of the combined cofferdam. The combined cofferdam comprises a longitudinal concrete cofferdam body and twotransverse soil-rock cofferdam bodies, wherein the two transverse soil-rock cofferdam bodies are arranged between the two ends of the longitudinal concrete cofferdam body and a river bank. The construction method comprises the steps of construction of a temporary cofferdam, construction of the longitudinal concrete cofferdam body and construction of the transverse soil-rock cofferdam bodies. According to the construction method, a gabion facing layer and a stone facing layer are arranged on the upstream face of each transverse soil-rock cofferdam body, the water flow scouring resistance of the upstream faces of the transverse soil-rock cofferdam bodies is improved, and the phenomenon that a water body is polluted by muddy water because of scouring of rainwater and water flow is avoided. In the construction method, a bottom blocking structure and a vertical water-proof structure are directly constructed at a time, and due to the construction of a first seepage-prevention wall, the impermeability of the concrete cofferdam body itself is effectively increased; the first seepage-prevention wall and a second seepage-prevention wall are constructed by using a high-pressure chemical churning pile device so that setting of mixed cement grout can be rapid, it is avoided that the mixed cement grout goes away with the water flow before setting, and therefore, the aims of solidification and seepage prevention are achieved.