The invention relates to a casting cable trench after assembly of L-shaped precast concrete and a construction method of the casting cable trench. A cable trench comprises a plurality of precast concrete which is symmetrically arranged in two rows in a U shape, water swelling water stop bars are mounted in semicircular grooves among adjacent L-shaped precast concrete, adjacent L-shaped precast concrete is connected through bolt holes formed by bolts penetrating through iron parts, connection sections of floor stress bars between the two rows of the L-shaped precast concrete are mutually welded, the floor stress bars are symmetrically arranged, concrete layers are casted into areas of floors of the two rows of the L-shaped precast concrete, the thickness of the concrete layers is not larger than the height of the floors on two sides, and drainage pipes are arranged on the concrete layers at intervals. Compared with existing U-shaped precast concrete, the L-shaped precast concrete has better adaptability and flexibility, and the manufacturing cost, the transportation cost, the hoisting cost and the maintenance cost are lower.