The invention relates to a beam and an application method thereof, in particular to a precast structural beam and an application method thereof, and belongs to the field of building structures. A finitely deconstructed precast convex beam is arranged on a column, and the convex beam comprises a lower unit component and an upper unit component; the lower unit component is of a pretensioned prestressed structure and is formed by precasting prestressed reinforcement and concrete, and a stirrup is arranged in the axial direction of the lower unit component, and the end of the stirrup extends out of the upper end face of the lower unit component and serves as one part of the upper unit component; the upper unit component is of a rear cast-in-place structure and is composed of reinforcement, a stirrup and concrete, and the reinforcement and the stirrup are arranged according to the design requirements; the size of the upper unit component is smaller than that of the lower unit component. Thebeam has the following advantages that the overall rigidity of the building structure space is improved; the stiffness of the components is enhanced, and the technical limitation of the height span ratio of ordinary reinforced concrete beams is broken; the gap between the plate thickness and the beam height is reduced, and the plate thickness and the beam height tend to be consistent; the space layout is more flexible.