The invention discloses a supporting device and a construction method for dredging engineering of plain river courses, comprising a fixing seat, a supporting frame, and a supporting mechanism, a positioning bar limiting a fixing rod, a fixing block provided on the fixing rod, and a fixing groove provided on the fixing block. The fixing seat is provided with connecting blocks, and the connecting blocks on both sides of the fixing block are provided with fixing bolts, and the fixing bolts are arranged in the fixing grooves, and the fixing bolts limit the fixing blocks, the supporting frame is provided with a supporting rod, and the supporting frame is provided with a ferrule, and the supporting frame is provided with a ferrule. The rod is provided with a card rod, the card rod is provided with a ferrule, the support rod is provided with a guide block, the support rod is provided with a chute, and a water retaining mechanism is provided between adjacent support rods; the method adopts six steps. The invention can block river water, facilitate the dredging and cleaning of cross-river roads, effectively shorten the construction period, have simple steps, facilitate construction, effectively improve construction efficiency and construction effect, and shorten construction period.