The invention provides a protecting device for an upper port of a guide wall of a concrete diaphragm wall and a protecting method thereof. The protecting device comprises a plurality of protecting roof units detachably connected end to end. Each protecting roof unit comprises a width adjustable supporting frame and a tunnel, wherein two sides of the supporting frame are separately overlapped to guide walls on two sides of the concrete diaphragm wall; the tunnel and the supporting frame are arranged in a same direction, the tunnel comprises two meshes hinged and connected together, the two meshes are separately provided with opposite hinge sides and free sizes, and the free sides of the meshes are separately connected to two sides of the supporting frame. The protecting device solves the problem that staff and articles are likely to fall off at the upper port of the constructed guide wall of the concrete diaphragm wall in construction of the concrete diaphragm wall and the like.