The invention discloses an inverted-U-shaped component for an underwater tunnel and a construction method of the inverted-U-shaped component. Tracks are arranged at the bottoms of two side walls and on the front end surfaces and the rear end surfaces of the two side walls of the inverted-U-shaped component 1; chain ditchers operate on the tracks during construction; the chain ditchers, transmission devices and a top machine frame form a digging device; the transmission devices are composed of driving devices, chain wheels and the like and installed on the top machine frame, and the top machine frame is located above the inverted-U-shaped component 1; and during construction, the chain ditchers on the tracks located at the bottoms of the two side walls of the component 1 dig rock and soil, the chain ditchers operating from bottom to top transport the dug rock and soil to the upper portion, and the chain ditchers at the bottoms of the two side walls of the component 1 and the component 1 descend together. The inverted-U-shaped component for the underwater tunnel and the construction method of the inverted-U-shaped component have the advantages that the inverted-U-shaped component is provided with the digging device, thus, the inverted-U-shaped component can be prefabricated sectionally in a factory, and construction is easy; digging and construction spaces inside the inverted-U-shaped component are defined by the inverted-U-shaped component, retaining walls and bottom slabs, inner rock and soil in the multiple construction spaces are removed, floor slabs or the bottom plates are poured, the construction earthwork volume is small, and construction is easy.