There are different degrees of cracks in prestressed concrete (PC) beams, especially the transverse cracks in the bottom plate and the vertical cracks in the web of prestressed concrete box girders, which will reduce the bearing capacity of the structure. In order to clarify the evolution law of cracks in prestressed concrete box girders after cracking, a 25 m prestressed concrete box girder was poured and a full-scale model test was carried out. At the 1/4 span position of the test beam, the test beam was cracked in 19 steps to 1900 kN. Then at the 3/4 span position, the test beam was cracked in 18 steps to 1800 kN, and after that the bending load test was carried out in 17 steps to analyze the influence of existing cracks on the bending cracks of the test beam. Combined with fractal theory, the evolution law of cracks, the development law of fractal dimensions, and their relationship under different loading conditions were analyzed. The results showed that the crack distribution of the test beam satisfies self-similarity and had fractal characteristics in a statistical sense, which could reflect the process of crack occurrence, propagation and damage, and could quantify the crack shape. The fractal dimension of cracks gradually increased with the increase in concentrated load on the test beam, and the fractal dimension of cracks had a linear relationship with the concentrated load. The crack propagation process of the concrete beam reflected the fractal dimension increasing process; that is, the crack damage evolution process of test beam was a dimension increasing process, and the fractal dimension of crack distribution increased linearly with the applied load. The functional relationship between prestressed concrete crack width and fractal dimension was constructed, which provided a new path for bridge safety evaluation. The research results provide engineering reference for the cause analysis and maintenance treatment of similar bridge cracks.