The invention discloses a double-layer composite truss strengthening type steel cofferdam. The double-layer composite truss strengthening type steel cofferdam comprises a steel cofferdam body. A double-layer truss structure is connected to the inner side of the steel cofferdam body. Steel pipe piles are arranged in the double-layer truss structure. The double-layer truss structure and the steel pipe piles are connected together through connecting steel pipes. A construction method of the double-layer composite truss strengthening type steel cofferdam comprises the following steps that firstly, the steel cofferdam body is spliced, and the front face of the steel cofferdam body is installed on the upstream face; secondly, the steel pipe piles are inserted and driven in drilled piles already constructed in the steel cofferdam body in advance; thirdly, the double-layer truss structure is installed in the steel cofferdam body; fourthly, water is injected in the steel cofferdam body, and then the steel cofferdam body sinks; and fifthly, the double-layer truss structure and the steel pipe piles are welded together through the connecting steel pipes. The double-layer composite truss strengthening type steel cofferdam is simple in structure, quick to install, capable of completely meeting the water closing requirement and high in structural strength and stability.