The invention provides a rapid construction method for a U-shaped groove crossing an existing culvert, and belongs to the technical field of culvert U-shaped groove construction. The rapid construction method comprises the following steps that a U-shaped groove foundation pit is dug; supporting piles are applied; a U-shaped groove section culvert structure top plate is broken, the pile tops of theconstructed supporting piles are removed, and pile top anchoring crown beam steel bars are reserved; full-hall supports are erected in the culvert; a pile top crown beam, a structural distribution beam and co-building structure steel bars of a U-shaped groove bottom plate and the culvert top plate are applied; and after the co-building structure of the U-shaped groove bottom plate and the culverttop plate is cast and formed at a time by adopting C35 concrete, wall bodies on both sides of the U-shaped groove are constructed through pulling a vertical formwork in opposite, and a U-shaped groove wall rear soil body is backfilled after the strength is achieved. According to the rapid construction method, on the premise of ensuring the normal using function of the existing culvert and reducing the influence of peripheral pipelines, the construction efficiency of the U-shaped groove is improved to the greatest extent, and the construction process is simplified.