The hydration heat was tested to investigate the early crack in the web of a long-span PC box girder bridge. Based on the mechanical properties of specimens cured at the construction site, the temperature and stress fields were simulated in time varying model by finite element method. The prestress tension was analyzed with construction stage considered as well.The results demonstrated that the calculated results of hydration heat agreed well with the measured ones. The hydration heat is one of the main reasons for the early cracks in the web of box girder, while the stress of the concrete box girder is at a very low level during prestress tension.