In conventional bridge health monitoring, a number of sensors are deployed on a bridge directly for detecting its various dynamic properties. This is so called “direct method”. But the drawbacks of the direct method are: laborious deployment of sensors, time-consuming, and not portable. Following the previous Yang’s works (2004) in indirect method by using a passing test vehicle as a message receiver of bridge response, this study regards cross winds as lateral excitational sources to detect the lateral bridge frequencies from the lateral response of the moving test vehicle. To account for the wind-vehicle-bridge interactions in performing dynamic analysis, an iteration-based 3D vehicle-bridge interaction (VBI) finite element method is developed. The whole wind/VBI system is decomposed into two subsystems: the wind-bridge subsystem and the wind-vehicle subsystem. Then the iterative scheme is carried out to compute the interaction response between the two subsystems independently and iterate for removing unbalance forces. The numerical results indicated that the present indirect bridge monitoring is a simple and feasible method to measure the lateral frequency of a long-span bridge in cross winds.