Using a two-dimensional flow model around a circular cylinder, the lift and drag functions of a straight tube heat exchanger were developed to estimate the external flow action with a wide range of Reynolds numbers. Subsequently, an improved model coupling the internal flow with the external flow was proposed, in which the lift and drag functions were treated as excitations and the added mass resulting from the unsteady internal and external flows was also considered. The results showed that the added mass due to the cross flow led to a drift in the natural frequency of the tube and that the Reynolds number also affected the frequency of the external excitation. Given that the internal flow Reynolds number also affects the natural frequency of the tube, resonance can occur under the combined action of specific internal flow and cross flow. The current model can predict the range of internal and external Reynolds numbers and therefore provide a theoretical basis for reasonably setting the safe operation conditions of internal and external flows.