In order to achieve rapid and accurate testing of the longitudinal loads of open-top wagons for heavy haul, the theory of grid-insensitive structural stress method was introduced. A calculation method for dynamic structural stress under multi-channel dynamic excitation was proposed, and a scheme for finding strain measurement points of the longitudinal load of the vehicle body based on modal decoupling was formed. The dynamic response test during the running on the railway line was carried out. Taking the data of the tested dynamic strain measurement points as the simulation target, the longitudinal dynamic load of the vehicle body was identified by using the load inversion method with time-domain iteration. The results show that: for the C70E open-top wagon, the modal decoupling measurement points of the longitudinal load are located behind the center plate of the vehicle body, with a horizontal distance of 115 mm from the small cross beam on the floor and a vertical distance of 135 mm from the upper cover plate of the center sill; the longitudinal load inverted under the dynamic excitation system has the same waveform as the longitudinal load tested by the traditional coupler. Due to the absence of the influence of the coupler buffer, the coupler force load obtained by inversion under the dynamic excitation system is more accurate than that of the traditional coupler testing method. The longitudinal load inversion method based on the vehicle body strain can conduct rapid testing on the premise of ensuring accuracy, providing an important means for the longitudinal load testing of railway freight vehicle bodies in the future.