Recent years, the failure of the upper arm has occurred frequently. Inspired by the structural failure of the pantograph, the load asymmetry of the pantograph upper arm is found and the reason is analyzed based on the test in laboratory. What’s more, a methodology is proposed which can be used for efficiently calculating the spatial load of internal hinge joint of the pantograph, while also considering the dynamic calculation of the pantograph catenary systems dynamics interaction. Firstly, the multi-rigid body model (MRBM) is established, secondly, based on the MRBM, and the modal reduction, the degrees of freedom of upper arm and collector head guidance are improved to effectively reflect the spatial loads. Finally, validation of the improved model is performed by comparing it with widely used MRBM of pantograph. The improved model aims to obtain the contact force of pantograph-catenary system and internal loads of the pantograph, which can be used for calculating the fatigue strength of the internal structure and compiling the load spectrum.