The invention discloses a deep-water foundation construction method for a large-span continuous beam adjacent to an existing railway line. A deep-water foundation for construction comprises an underwater abutment and a plurality of drilled piles, wherein the underwater abutment is positioned on an underwater base rock on one side of the existing operating railway line, and the drilled piles are used for supporting the underwater abutment. The deep-water foundation construction method includes the steps: firstly, excavating an underwater foundation pit mounted at the bottom of a cofferdam by setting up an excavation platform, mounting an impact drill, moving the excavation platform and performing first impact, synchronous slag removal, second impact and synchronous slag removal; secondly, constructing the drilled piles and the underwater abutment by surveying and setting out, lowering a cofferdam, mounting a steel casing, sealing the bottom of the cofferdam, constructing the drilled piles, drawing water in the cofferdam, cutting off the steel casing and constructing the abutment. The steps are simple, the method is reasonable in design, simple and convenient in construction, good in use effect, safe and reliable in construction process and less in external influence, and the deep-water foundation construction process can be simply, conveniently and rapidly finished.