The invention relates to a cable trench structure laid on a power transmission line, in particular to a power transmission line L-shaped assembled cable trench and a laying method thereof. The cable trench is characterized in that the cable trench comprises plural trench bodies connected end to end successively; each trench body comprises two L-shaped prefabricated members that are spliced face to face into a U-shaped trench body; a plurality of splicing bolt holes are uniformly and longitudinally distributed on the bottom horizontal plate of each L-shaped prefabricated member; horizontal bolts are connected to corresponding splicing bolt holes to achieve fixed connection after the two L-shaped prefabricated parts are butted face to face; the front end face of each trench body is provided with a tenon, the rear end face of each trench body is provided with a mortise, the tenon is connected with the mortise on the rear end face of a previous trench body and the mortise is connected with the tenon on the front end face of a following trench body so that an end-to-end structure is formed; and water-swellable rubber bars are pressed in gaps in the mortise joints. The cable trench decreases the work amount of construction, shortens a construction period, increases the efficiency of long-distance transportation, reduces construction cost, and is expanded in application range in transmission line projects.