In order to accurately assess the operational safety level of existing bridges under the continuous growth of traffic volume, a time-varying feature of the improved generalized extreme value distribution (GEV) based on “time division-extreme probability accumulation” is proposed, and the applicability of the method is verified by a typical example. Based on WIM measured traffic data, the maximum bending moment of existing arch bridges under traffic volume growth and overload rate limit is predicted. The results show that the increase of traffic volume leads to a significant increase in the extreme value of the on-board effect of the bridge. When the annual traffic growth rate is 2%~4%, the maximum bending moment of the existing arch bridge increases by 10%~15%. When the annual traffic growth rate is 4%, the maximum bending moment of the arch bridge will be greater than the design standard value, and the traffic volume of the bridge should be controlled. The overload rate limit has a significant impact on the extreme value of the on-board effect of the bridge. The limit of overload rate has a significant impact on the extreme value of bridge vehicle effects. When the upper limit of overload rate is 50%~200%, the maximum bending moment is reduced to 70%~82%.