In this paper, a coupling longitudinal-vertical dynamic model of a wheeled vehicle under off-road conditions is established. In the present model, the vehicle’s travelling velocity changes as the vehicle heaves under ground excitations. The relative movements between the vehicle body and the wheels in both longitudinal and vertical directions are taken into account. From this model, the vehicle’s vertical and pitch motions as well as the longitudinal dynamics are studied. For validation purposes, an experiment was carried out on a wheeled vehicle as travelling through a single speed bump. Through comparisons, it indicates that the proposed model performs well in predicting the dynamic characteristics of the vehicle on an uneven road. Thus, the model is further applied to quantitatively investigate the energy consumption of a vehicle under off-road conditions. It has been found that the energy consumption distribution is strongly affected by the vehicle speed and the road conditions. Both a higher vehicle speed and a larger road roughness coefficient lead to a lower drive power efficiency and a greater power loss.