Deep foundation pit combination support close to existing building and construction method thereof

The invention provides a deep foundation pit combination support close to an existing building and a construction method thereof. The combination support comprises multiple cast-in-situ bored piles, and multiple steel plate piles. The cast-in-situ bored piles are arranged between the existing building and a deep foundation pit, and are arranged in a strip manner in the extending direction of the edge of the existing building, and a first blocking layer is formed; the steel plate piles are driven along the periphery of the deep foundation pit, a second blocking layer is formed, and a first supporting structure serving as a transverse support is arranged on the inner wall of the second blocking layer; the first supporting structure comprises a first surrounding purlin which is horizontally arranged along the inner wall of the second blocking layer, and at least one transverse support is arranged on the first surrounding purlin; and the length of the first blocking layer and the second blocking layer is not smal