The invention discloses an L-shaped partially-prefabricated composite beam, and a construction method and application thereof, and belongs to the technical field of buildings. The composite beam givesfull play to the performance of steel and concrete materials. Concrete is packed in profile steel, the constraint effect on the concrete is enhanced due to the existence of the profile steel, and thepossibility of local buckling or overall instability of the profile steel is reduced due to the existence of the concrete, so that the bearing capacity of the composite beam is improved, and meanwhile, the purpose of reducing the section size of the composite beam is achieved. The web plate of the composite beam is a corrugated plate with higher shear strength, and the constraint effect on concrete is greatly improved due to higher lateral rigidity of the web plate. Aiming at the bonding among second precast concrete, cast-in-place concrete and H-shaped steel, the composite beam adopts a modeof punching a shear key on the web plate of the H-shaped steel, so that the shear key is partially arranged in the second precast concrete, and the shear key is partially arranged in the cast-in-place concrete, thereby ensuring co-working of the concrete and the concrete and co-working of the steel and the concrete.