The utility model discloses a connecting structure for a large-diameter lamp post and an existing bridge anti-collision guardrail rod piece foundation. The anti-collision guardrail comprises an anti-collision guardrail body, a new lamp pole and a new lamp pole flange plate. An existing pre-buried steel plate is pre-buried in the top of the anti-collision guardrail. A bolt conversion steel plate isadditionally arranged between the new lamp post flange plate and the existing embedded steel plate; a new lamp post flange plate bolt hole and an existing foundation bolt hole are formed in the surface of the bolt conversion steel plate; according to the utility model, the bolt conversion steel plate is additionally arranged between the new lamp post flange plate and the existing embedded steel plate; the bolt conversion steel plates are arranged to correspond to new lamp pole flange plate bolt holes and existing foundation bolt holes respectively, a new street lamp pole is indirectly fixed to existing foundation bolts through the bolt conversion steel plates, the functions of the existing foundation bolts are brought into full play, and foundation transformation is avoided. Side steel plates are welded to the edges of the bolt conversion steel plates and fixed to the side faces of the crash barrier through anchor bolts, and the foundation stability is enhanced.