According to measured traffic flow statistics from the Nanjing Third Yangtze River Bridge, in Jiangsu Province, a vehicle load spectrum in a random state was developed based on the theory of fatigue damage equivalence, and a full-bridge dynamic response analysis was conducted using finite element numerical simulation. Through calculation of the displacements and vibration response time-history curves of internal forces in different sections of the main beam of the bridge under the influence of measured vehicle flow, the changes of internal forces and dynamic magnification factors were analyzed. The results show that, under the influence of random traffic, the absolute values of the bending moment and axial force were at their maximum at the bottom of the tower of the main beam. The mid-span bending moment and axial force changed most significantly. At one quarter of the span, the displacement vibration was most intense. The dynamic magnification factor ranged from 1. 00 to 1. 03 at the maximum stress amplitude at mid-span, one quarter of the span, and near the tower bottom. The dynamic magnification factor was more stable than in the case considering only the bending moment or the axial force, and was consistent with that of the current specifications in China.