The invention discloses a pre-tensioning method prefabricated I-shaped beam and a construction method thereof. The construction method comprises the steps of steel strand tensioning and releasing, steel reinforcement framework binding and hoisting, concrete pouring and maintenance and I-shaped beam form removal. According to the method, effective prestress is guaranteed by collecting and controlling the tensile stress of the steel strands, and the shrinkage creep model of the pre-tensioning method I-shaped beam is established according to the actual attributes of poured concrete and a water reducing agent, so that the construction scheme and the structural design are optimized; the prestress is transmitted according to the bonding force of the steel strand and the poured concrete; in the elastic stage of the whole structure, the prestress is transmitted into the concrete structure through a certain transmission length; meanwhile, the steel bundles formed by the steel strands are free of frictional resistance loss along the way in the prefabricating process, so that the engineering quality is guaranteed, and the problems that the overall structural strength of the I-shaped beam is greatly reduced due to the fact that the prestress friction of the post-tensioned prefabricated part is too large, the stress ribs are prone to retraction and the prestress loss is too large are solved.