The prestressing load is one of the key loads in P.C.bridge under normal service stage. In modern P.C.bridges, besides the longitudinal prestressing forces, the transversal and vertical prestressing forces are often used. The spatial stresses effect caused by prestressing force is very evident and the behaviour of structure is complicated in the P.C.bridges. The traditional elementary beam theory can not analyze the spatial stresses effect. Based on the conventional 3D isoparametric finite element method, one new, more convenient and effective element—degenerated solid element for analyzing all kinds of complicated structures is presented. It assemble the advantages of the shell element and 3D isoparametric element. In the end, a typical three-span P.C. continuous rigid-frame box girder bridge is analyzed by the method. Influence of prestressing force on spatial effect is explored. The analysis results indicate that the degenerate solid element is a effective and practical method.