To realize the dynamic analysis of expansion joints under traffic flows across bridges， aiming at the large size difference between the main beam and the expansion joint in the bridge-expansion joint system， the complex structure of the expansion joint， and the problem that the conventional modeling approach falls short in balancing detailed analysis with calculation efficiency， a “cross” modeling concept of bridge-joint integration was proposed and a vehicle bridge joint analysis system was established to investigate the influence of dynamic factors of traffic flow on the dynamic response of expansion joints. Firstly， on the basis of clarifying the internal structure of the expansion joint and the movement correlation of the component， the longitudinal division of the bridge element， transverse division of the expansion joint element and multi-point constraint organic connection of the beam expansion joint were proposed. Secondly， a vehicle-bridge-joint analysis program was connected through the program call and preparation of the connection program. Finally， based on the traffic load survey data， the typical traffic flow load conditions were constructed， and the influence of traffic flow dynamic factors such as vehicle weight， speed， and vehicle distribution on the dynamic response of car-joint structure was explored. The results show that： 1） The bridge joint integrated finite element model established based on the “cross” modeling concept not only meets the computational efficiency but also takes into account the detailed components of the expansion joint. 2） There is a significant positive correlation between the vertical displacement of the main beam at the midspan of a cable-stayed bridge and the weight of moving vehicles on the bridge， while the correlation with vehicle speed is weak. The spacing between vehicles in the train decreased from 50 meters to 30 meters， and the maximum vertical displacement at the midspan of the main beam increased by about 34%. The more concentrated the vehicle load on the bridge， the greater the vertical displacement at the midspan of the main beam. 3） The cumulative sliding stroke of the middle beam from the beam end to the fixed end of the expansion joint gradually decreases. Under single vehicle operating conditions， there is a positive correlation between vehicle speed， weight， and longitudinal displacement of expansion joints. There is a negative correlation between the maximum longitudinal displacement of the expansion joint and the distance between vehicles during train operation. The impact of vehicle braking on the dynamic response of the expansion joint is very significant， and the longitudinal displacement response of the expansion joint 1 # beam is 3.97 times that of the normal sports car working condition.