The invention relates to a dam foundation deep groove processing structure and a construction method thereof. The invention aims to provide a dam foundation deep groove processing structure and a construction method thereof. The dam body construction above a dam foundation deep groove is not affected when the dam foundation deep groove is processed; the deep groove processing and the dam body construction are performed at the same time, so that the problems of stability, stress, deformation and seepage prevention of the dam foundation caused by sand gravels in the deep groove are solved radically, the processing period of the deep groove is also shortened and the project benefit is also played in advance. The technical scheme of the invention is as follows: the dam foundation deep groove processing structure is characterized in that a bearing plate is arranged to cross through the dam foundation deep groove, the upper part of the bearing plate is provided with the dam body, the top part of the bearing plate is obligated with a construction passage, a grout pipe which communicates the construction passage with the bottom of the bearing plate is pre-buried inside the bearing plate, and deep groove replacement concrete is backfilled in the dam foundation deep groove at the bottom of the bearing plate. The invention is suitable for projects having dam foundations existing deep grooves.