The invention discloses a lengthening construction method for improving bearing capacity of an existing pier or foundation pile. The lengthening construction method for improving the bearing capacityof the existing pier or foundation pile is characterized by including following steps: performing drilling and hole forming operation in an existing pier foundation so as to form a vertical small pilehole; and arranging a small pile core material in the small pile hole, injecting a cementing material into the small pile hole in a pressing injection or pouring injection mode so as to form a smallpile, and cementing the small pile and the pier foundation into an induration body. The lengthening construction method for improving the bearing capacity of the existing pier or foundation pile has the advantages of fully using the constructed original pier or foundation pile, performing effective lengthening and reinforcement by constructing the small pile in the constructed original pier or foundation pile, and realizing expected design bearing capacity or on-demand rock embedding requirements; avoiding resource waste to the utmost, and reducing environment pollution; and achieving cross streamline operation by combining small pile construction with other construction contents, and effectively saving construction time.