Double-wall steel cofferdam has become an ideal enclosure structure in deep-water foundation construction due to its virtue of high strength, high rigidity and good waterproof performance, and has been applied in bridge construction on a large scale. This paper takes the construction of Pier 5 bearing platform of Leishui Large-Span Bridge with water depth of 18 m as the engineering background, and introduces the design of double-wall steel cofferdam for deep-water bearing platform and the design parameters of double-wall steel cofferdam with height of 20.8 m. The paper adopts Midas/Civil to establish finite element model for the cofferdam structure, to check the strength, stiffness and stability of the cofferdam structure. The paper gives a detailed introduction to the checking of the cofferdam’s anti-floating, anti-slumping, and key weld joints, etc. In addition, for the methodology of piling before cofferdam construction, the paper introduces the design and technology of the first section assembly platform and lifting system of the cofferdam.