The prestressed concrete box girders was widely used in highway and urban bridges owing to its many advantages. However, concrete box girders cracked commonly in period of construction and service. The cracks have affected safety , durability and service life of bridges. It is quite indispensable to giving the reasons of cracks. Reviewed and evaluated research theories and methods of concrete cracking, numerical method to analyze cracks in PC box girder bridges was applied. The modification of current design method was brought forward. The main contents of this dissertation are described as follows:· Through a literature review process and a comprehensive nationwide survey, this paper includes a thorough investigation to identify the probable causes of cracking in prestressed concrete box girders, particularly about position, modality and distribution of the cracks. Based on the computing theories and methods of cracking, The ideas to resist cracking was brought forward. To study the problem, the numerical method of compounding elements was developed.· Generated a finite element mesh in existence numerical method, node superposition of concrete and reinforcement element is difficult to curved tendon box girder. To find a solution, the concrete and reinforcement steel is respectively represented by isoparametric 20-node brick elements and embedded reinforcing bars. Neglecting bond slip, the stiffness matrix of embedded reinforcement elements that contains the contributions of the reinforcing bar is derived. The embedded representation of the reinforcement allows generating a finite element mesh without taking much care about the layout of the reinforcement. Rather, the reinforcing bar may pass through the concrete elements in an arbitrary manner. by means of transition matrix of displacement, the forces exerted from the tendons on the adjacent concrete elements and the prestress losses caused by creep and shrinkage of concrete is realized.· Adopted embedded reinforcement elements, It is essence difference to simulate the mechanics property of box girder. Applied embedded reinforcement elements and taken into account concrete creep, a program of PC box girders was