To accurately calculate the prestress of externally prestressed composite girder bridges, the six critical factors (the prestressing tendon withdrawal and anchor deformation, friction between prestressing tendon and deviator, prestressing tendon relaxation, concrete creep, concrete shrinkage and temperature changes) that cause the prestress loss of such type of the bridges are summarized and the corresponding simplified calculation methods are respectively derived on the basis of the existing researches. The prestressing tendons ability has an important influence on the mechanical behavior of prestressed composite girder bridges, which is the key design parameters. Prestress loss will occur in the process of long-term use, so that the whole beam stress redistribution occurs. How to accurately calculate the value of the prestressing loss is an issue of great concern to engineers. And at present there is few research for prestressed composite girder bridges. On the basis of existing research, this paper summarizes the key factors that lead to loss of prestress and derives the corresponding simplified calculation method for design reference.