The invention discloses a fully-assembled modular prefabricated cable trench, which includes prefabricated main trench frame bodies and lightweight trench sections. Each main trench frame body includes a U-shaped main body and connecting lug plates symmetrically prefabricated at both ends thereof. Each lightweight trench section has a cross section in an inverted trapezoid shape with an upward opening. The connecting lug plates are used as the connection structure of the lightweight trench sections and have shapes matching that of the lightweight trench section. The end portion of each lightweight trench section is connected to the connecting lug plates by fasteners and sealing strips. Each U-shaped main body is used as a main bearing member, and each lightweight trench section is used asa cable trench length splicing main body. The U-shaped main body has a wall thickness greater than the wall thickness of the lightweight trench section, and a length less than the length of the lightweight trench section. The plurality of lightweight trench sections and the main trench frame bodies are connected successively to form the prefabricated cable trench. Both the weight of the main trench frame body and the weight of the lightweight trench section are significantly reduced. The construction operation of the main trench frame bodies and the lightweight trench sections is convenient during field installation, which improves installation efficiency and simplifies the construction process. The lightweight trench sections can be stacked up and down during transportation, thereby reducing transportation cost and improving transportation efficiency.