In order to solve the problem of continuous downward deflection of the main girder for a concrete cable-stayed bridge during its service life，the deflection variation of the main girder for a long-span concrete cable-stayed bridge during its service was analyzed based on the modified model.A dual-objective optimization problem was constructed by using the combined static and dynamic finite element model modification method and was solved by the non-dominated sorting genetic algorithm （NSGA- Ⅱ）.The Pareto optimal solution set was obtained，and the coordinated optimal solution was found in the Pareto optimal solution set，so as to modify the finite element model.The calculated values of static displacement and natural frequency of vibration were in good agreement with the measured values after model modification，which can appropriately reflect the actual working state of the structure.On this basis，the influence of time-varying factors such as creep mode and cable relaxation effect on main girder alignment was analyzed by referring to the monitoring data over the years.The results show that the deflection calculated by CEB-FIP 2010 creep mode is closer to the actual deflection.The largest downward deflection is located in the middle span of the bridge，which is about 270 mm after 20 years of service.In the first five years，the average growth rate of the mid-span deflection of the main girder is 33 mm per year，and the total deflection in the first five years accounts for about 50% of that in the first 40 years.The downward deflection of the main girder tends to be gentle in the later period.