Abstract: On October 10, 2019, a three-span concrete continuous box girder on National Highway 312 in Xishan District, Wuxi City, overturned under the action of two vehicles. Investigated the accident site first, then established a geometrically nonlinear finite element model based on wreckage photos and construction drawings, and studied the overturning mechanism. An overload determination method based on the strength and overturning stability effects is proposed. The results include: the overturning test results based on the standard lane load demonstrate that the bridge does not meet the overturning test requirements specified in the Specification for Design of Highway Reinforced Concrete and Prestressed Concrete Bridge and Culvert (2018 edition); the superstructure under the action of the overloaded vehicle first rotated, then slid, and finally exhibited a combination of substantial rotation and sliding overturning mode; the superstructure has a certain safety margin between bearing separation and overturning; the calculation based on the strength and overturning stability effect proves that the overturning effect generated by the overloaded vehicle is greater than the overturning capacity of the structure, indicating that the overturning is mainly attributed to the overload. However, the structural bending capacity turns out to be sufficient through further calculation, indicating that the overturning damage occurred earlier than the occurrence of strength damage, and that the overturning resistance and bending resistance do not match.