In the process of bridge construction, the cast-in-situ box girder formwork support system is a sub-project with greater risk, and a little carelessness in the construction process can easily cause all kinds of safety accidents, and even system collapse accidents, causing mass deaths and injuries. Combined with the relevant specifications and the whole life cycle theory, the construction process of the formwork support system is analyzed. Referring to the past accident cases, the causal factors are analyzed. L-S and AHP are used to calculate the weights of each influencing factor index, and the safety degree of the formwork support system is quantitatively evaluated with the help of actual project construction cases. Finally, based on the results of the ranking of all causal factors, evaluate the risk level of different causal factors and formulate targeted countermeasures in the light of the actual situation, in order to put forward reasonable suggestions for the risk management and control of the formwork support system in the construction process.