Abstract: The equations of unsteady aerodynamic forces of the body emerged in fluids were derived from the basic equations of fluid mechanics. The equations were suitable not only for single connected fluid domain around bridge decks but also for multiple connected fluid domains around multiple separated box girders after using the Ostrovski-Gauss formula to change the integral from volume to surface. At the same time, the simplified equations were obtained for the sample rate of flow maps is low. The flow maps around the square cylinder were obtained in the Reynolds number of 22,000 using both numerical simulation and Particle Image Velocimetry which coincide with the results of experiments carried by other researchers. The aerodynamic forces were obtained using the flow map to validate the equations. The results show that it has a good agreement in damping force coefficient but poor in lifting force coefficient. When the equations were simplified to neglect the velocity difference in time, the results agree well. While the equations were simplified further to use only limited velocity data around only one segment, the results differs greatly with the results from original equations. Furthermore, the flow maps around the twin box girders from numerical simulation were also used to validate the equations. The results were the same with that of square cylinder, that is to say, good in damping force coefficient but poor in lifting force coefficient. At last, the reason of such discrepancies was analyzed.