Cracking　in　concrete　box　girder　bridges　will　have　a　significant　impact　on　the　safety　and　durability　of　the　structure,　and　many　box　girder　bridges　which　are　in　service　have　undergone　varying　degrees　of　cracking.　Currently,　the　safety　design　of　actual　bridge　projects　place　an　emphasis　on　the　stress　or　the　load　value　of　a　cross　section　at　the　limit　value　specified　in　the　code　for　safety　control.　This　design　method　assumes　that　the　member　itself　is　of　uniform　and　continuous　material　and　is　internally　undamaged.　However,　the　bridge　structure　is　more　or　less　cracked　to　varying　degrees　during　the　period　from　casting　to　construction　to　operation　of　the　concrete　members.　In　this　paper,　a　finite　element　computational　model　of　a　three-span　prestressed　concrete　box　girder　bridge　with　existing　cracks　is　established　based　on　the　fracture　mechanics　theory,　and　the　critical　parameters　of　crack　extension　are　introduced　to　evaluate　the　extension　state　of　cracks.　At　the　same　time,　the　extended　stability　of　the　existing　cracks　of　the　box　girder　bridge　is　analyzed　by　considering　the　temperature　effect,　vehicle　loading,　and　prestressing　loss,　and　the　sensitivity　of　crack　extension　under　each　working　condition　is　investigated.　The　results　show　that,　with　the　increase　in　crack　length　and　depth,　the　crack　expansion　is　promoted,　but　the　effect　is　relatively　small,　and　the　maximum　stress　intensity　factor　is　only　6.48　MPa　mm1/2.　Under　the　multi-factor　coupling　effect,　the　cracks　show　a　composite　crack　expansion　dominated　by　type　I　cracks,　the　longitudinal　cracks　of　the　existing　base　plate　are　in　a　stable　state,　the　maximum　value　of　the　crack　expansion　critical　parameter　of　the　vertical　cracks　of　the　webs　reaches　1.087,　and　there　is　a　tendency　to　expand　locally.　The　maximum　value　of　the　critical　parameter　for　crack　extension　of　the　vertical　crack　in　the　web　plate　reaches　1.087,　and　there　is　a　tendency　towards　local　expansion.　The　crack　extension　evaluation　criteria　proposed　in　this　paper　have　a　certain　reference　value　for　crack　extension　research　on　similar　concrete　box　girder　bridges　and　provide　a　scientific　basis　for　the　optimized　design　of　similar　bridges.