The invention relates to a large cantilever bare concrete stair and a construction method thereof. The large cantilever bare concrete stair comprises stairsteps and stair platforms. One sides of the stairstep and the stair platforms are connected with a wall body; the other sides of the stairstep and the stair platforms are of cantilever structures; shear keys are arranged at connecting positionsof the stair platforms and the wall body and are multiple joist steel pre-embedded into the wall body. The construction method comprises the steps of setting the shear keys, bracing bare cantilever stair formworks, embedding pre-embedded parts, casting bare concrete, maintaining finished products and the like. According to the large cantilever bare concrete stair and the construction method thereof provided by the invention, special treatment measures are adopted for solving the problems of cantilever structure anchoring, bare structure formwork bracing and bear concrete casting, so that the safety of cantilever structures and an impressions effect of the bear concrete are ensured.