Composite boxed cofferdam and building method thereof

The invention discloses a composite casing box cofferdam and a construction method thereof. A double-wall steel cofferdam which consists of an inner wall and an outer wall which are connected is arranged on bedrock, positioned and fixed; mad and sand between the inner wall and the outer wall of the cofferdam are cleared; a bed rock hole is drilled on the bedrock between the inner wall and the outer wall of the cofferdam; steel reinforcement cages are arranged in the bedrock hole and between the inner wall and the outer wall of the cofferdam; bottom sealing concrete is poured into the bedrock hole and between the inner wall and the outer wall of the cofferdam, to construct the composite casing box cofferdam. The invention has the advantages of convenient construction, material saving, stable structure and high positioning precision.