The invention discloses a quick-installation comb plate extending device and a construction method thereof. The quick-installation comb plate extending device comprises comb plates, a rubber waterstop belt, a stainless steel sliding plate and an anchoring module, wherein the comb plates are composed of a small comb plate and a large comb plate; the rubber waterstop belt and a first pre-embedded leveling plate are arranged under the large comb plate, and the large comb plate, the rubber waterstop belt and the bottom of the first embedded leveling plate are connected through a first bolt. The quick-installation comb plate extending device is high in integral installation performance and installation precision and can improve the construction efficiency and reduce the installation cost; the pre-embedded leveling plate is provided with a comb plate mounting hole, an anchoring sleeve is arranged under the comb plate mounting hole, a U-type or L-type reinforcing steel bar is welded onto the bottom surface of the anchoring sleeve, so that the installation precision and the connecting strength of comb plate anchoring bolts can be improved, in-site anchoring bolt installation and positioning can be saved, and installation cost can be reduced; the horizontal plate of the pre-embedded leveling plate is provided with a vibrating exhaust port to ensure pouring compactness of the lower concrete of the embedded leveling plate under the installation position of the comb plates as well as the filling compactness of the concrete at the slot of the extending device, so that the installation quality can be improved.