Dropper is the key component of catenary in high-speed railways. The prediction of the fatigue life of dropper is accomplished using alternating load characteristics. Simulation method for obtaining the dynamic internal load of the dropper in previous studies was insufficient for accurate requirements in predicting dropper life. According to realistic working conditions in the simulation environment, this study proposed a simulation method considering horizontal vibration behaviour for the dropper. The horizontal vibration phenomenon of a dropper was determined on the basis of observing and measuring a real high-speed railway. Moreover, a test for the reappearance of horizontal vibration in the laboratory was performed. The dropper was modelled with multibody dynamics. Finally, the horizontal vibration behaviour of the dropper on-site was reproduced in a laboratory test and computer simulation. Results demonstrated that the coefficient of association between the virtual sensor load in a simulation environment and the real sensor load in a laboratory test is 0.9578. The proposed method provides an accurate approach to simulating the load of the droppers under real working conditions.