A FEA program using 8-node degenerated solid shell element was developed to analyze the spatial stress of long span prestressed concrete box girder bridges for the purpose of researching the relationship between cracking damage and the spatial stress state.The influences of dead load,box girder stiffness,effective prestress and temperature on the principal stress were researched,the reasonable magnification factors of live load stress were discussed and a two-axis strength criterion was proposed to control the concrete cracking of box girder.An analysis of real bridge shows that the FEA program is helpful to control the cracking damage of long span prestressed concrete box girder bridges.