Long span bridges are widely equipped with structural health monitoring (SHM) systems. Using SHM, a wide range of real-time structural responses under ambient loading could be recorded, which provides the basis for bridge load rating and structural performance assessment. During these structural responses, displacement is an important index to represent the bridge in-service condition. In the paper, monitored girder displacements of a cable-stayed bridge are examined to assess the bridge in-service condition. The finite element model of the studied bridge is first updated based on monitored natural frequencies. Realistic influence lines of the bridge could be obtained based on the updated model, which is the baseline for assessment. Then, moving average method and window smoothing method are employed to extract the displacement induced only by traffic loading from that by wind, temperature, and environmental noise. Finally, using the extracted traffic load effects of girder displacements, the characteristic values under various remaining lives are extrapolated through constructing the limit distribution using block maxima based generalized extreme value (GEV) model fitting. Results show the predicted displacements would exceed the design level of the Chinese code after the bridge serves for another 10 years, indicating potential risk. At that time, the management of traffic loads and maintenance of bridge components should be enhanced to ensure the bridge in-service condition.