Web cracking mechanism of long span rigid-frame prestressed concrete bridge construction stage was presented. Based on the elasticity theory, the 2-D stress function of web under patching load was deduced, the function curve of the horizon stress of different coefficient (d/a) was drawn, then a horizon stress function was fitted, and the method of equivalent pressure prism selection method was put forward. Later the 3-D situation horizon stress along the tendon line was considered based on 2-D horizon stress function. To verify the theory function, 0~3# rigid frame bridge segment finite element model was established, the concrete web cracking analysis was carried out in different prestressed level. The results showed that the crackling area of the web in the bridge was coincided with the finite element analyze, the lower limit stress of the finite analysis was also coincided to the theory calculated of transverse stress function put forward by this paper.