The invention provides a cable trench external corner fixation and protection device and method and relates to the field of power transmission line cable construction. The device includes a casing, ahorizontal pulley and a vertical pulley. The casing is a hollow structure with holes at both ends. A slot matching with a cable trench external corner structure is disposed outside the casing. The horizontal and vertical pulleys are installed inside the casing. The vertical pulley is installed above the horizontal pulley. The vertical and horizontal pulleys are in rolling contact with a cable. When in use, the slot is firstly matched with a cable trench external corner position; then the casing is fixed to an external corner; and finally the cable passes through the casing so as to be in rolling contact with the vertical and horizontal pulleys. The device and the method can take into account both protection and fixation of the cable, solve the problem of laying the cable at the corner of acable trench, and can effectively prevent the cable from being damaged at the external corner.