Belonging to the field of building materials and building construction technologies, the invention relates to a prefabricated reinforced concrete U-shaped frame beam and a construction method thereof. The prefabricated reinforced concrete U-shaped frame beam comprises a base plate (11) and side plates (12), and is characterized in that the base plate (11) is internally equipped with base plate longitudinal ribs (13), the side plates (12) are internally provided with anti-torsion structure longitudinal ribs (14), stirrups (15) are disposed around the beam, the base plate longitudinal ribs (13) stretch out of two ends of the beam, and in a direction over against a beam end, the first longitudinal rib from the right is longer than the other longitudinal ribs. The U-shaped frame beam provided by the invention can well form reliable connection with concrete in a post-poured groove, the integrality is good, the die is few and simple, and the construction is convenient. The U-shaped frame beam has very good strength and rigidity, does not crack in the lifting and transportation process. The base plate longitudinal ribs (13) stretching out from two ends of the U-shaped frame beam can be welded to a connection steel frame (50) at beam-column joints, thus reinforcing the anchoring performance of steel bars and also extending the anchoring length of steel bars. The prefabricated reinforced concrete U-shaped frame beam truly realizes assembly production, and has broad application prospect.