In order to study the relationship between the bearing capacity of the structure and the damage degree, the author proposed the bearing capacity parameter analysis and the heat storage loss test of the concrete composite box girder. Taking the five-span prestressed concrete composite box girder as an example, different degrees of damage are simulated by reducing the bending stiffness of different parts, and the modal strain energy method and flexibility difference method are used to diagnose the structural damage. The test results show that for the first span, the sum of the structure can meet the corresponding specification requirements under the condi­tion of dense traffic and less than 20% damage. When the damage is greater than 20%, the first span beam cannot meet the driving requirements. Under general traffic conditions, when the damage is less than 30%, the structure can basically meet the driving requirements, and the second and third spans still have a high safety factor. In the case of 30% damage of the first span, although β it does not meet the requirements, but RF > 0.9. Each monitoring point has good estimation accuracy, and the error between the maximum temperature occurrence time of each monitoring point and the maximum temperature occurrence time of the mon­itoring point is within ± 10 hours . For continuous composite box girder structure, the flexibility difference method is superior to the modal strain energy method.