To reasonably solve the durability assessment problem of in-service concrete beam bridges, a game theory-based fusion assessment model was proposed. First, considering the regional environment and the characteristics of the bridge itself, an index evaluation system was constructed. Then, based on the full consideration of the impact between indicators, the subjective weights were calculated according to the DEMATEL-ANP method, and the causal relationships between indicators were intuitively shown using the DEMATEL method. Next, the equilibrium theory of game theory was utilized to integrate the subjective weights and CRITIC objective weights, and combined with the uncertainty measure theory, the bridge grade was determined. Finally, the model was verified with multiple in-service concrete bridges. It is demonstrated by the findings that the durability of the bridges aligns with their actual conditions, yielding a high level of accuracy. Additionally, when compared to traditional evaluation models, this model is not only more accurate, but it also takes into consideration the influences among indicators and uses the causal relationships between indicators to offer a basis for the subsequent repair sequence, thereby better adhering to actual engineering requirements.