Provided is a totally-mechanical top-drive casing device by adoption of an externally-clamping mode. The periphery of a mandrel (14) is provided with a locking device used for clamping the outer wall of a sleeve pipe (17). The locking device comprises a slip internal cylinder (9), a slip external cylinder and a slip wedge block (11). The joint between the upper end of the mandrel (14) and an upper joint (1) is provided with an upper threaded sleeve (3). The upper end of the slip internal cylinder (9) is provided with a lower threaded sleeve (4). The outside of the slip wedge block (11) is provided with a wedge block body (18) having a guide oblique plane. The inner wall of the slip external cylinder is provided with a guide groove having a guide oblique plane. The upper end of the slip outer cylinder is connected with the upper joint (1). The slip wedge block (11) is located in a window of a cylindrical wall of the slip internal cylinder (9). The wedge block body (18) outside of the slip internal cylinder is nested into the guide groove of the slip external cylinder. A reset spring (10) is arranged between the slip wedge block (11) and the slip internal cylinder (9). The totally-mechanical top-drive casing device by adoption of the externally-clamping mode is advantaged by being reasonable in structure, easy and convenient in operation and great in load and is safe, reliable and highly-efficient.