The present invention discloses a monolithic prefabricated construction method of pretensioned beam, the main working procedures of which comprise: construction preparation, bottom die adjustment, guide device and lateral die installation, hoisting bottom web reinforcement, end die and steel beam installation, threading steel strand and PVC pipe, hoisting internal die and deck reinforcement, firstly pulling of steel strand, finally pulling of broken line reinforcement, finally pulling of straight line reinforcement, grider concrete packing, grider concrete maintenance, loosening internal and lateral dies, broke line extension, cutting guide device, straight line reinforcement extension, hoisting steel beam, end and internal dies removal, hoisting grider, cutting wire and sealing anchor, etc. The construction method adopts straight line and broken line reinforcements, which is able to meet the demand of prefabricated construction of bridge with 32m / 900t big span and large tonnage; at the same time, the construction method selects tension and extension technique of 52000kN large tonnage tensioning force, which ensures that the grider has balanced stress, and improves the tension and extension efficiency and security, etc.