In recent years, many low frequency oscillation of high-speed railway vehicle-network coupling system occurred in China. When the oscillation was severe, it even led to Electric Multiple Units (EMU) traction blockade. This paper presents an effective method to suppress the low frequency oscillation when the multi-CRH5 EMU pantograph is lifted and put into traction network. First, the characteristics of electric coupling system of multi-CRH5 EMUs simultaneously connecting to traction network are analyzed. Next, based on an Improved Active Disturbance Rejection Control (IADRC) strategy, the controller of CRH5 EMU rectifier load of the vehicle-network coupling system is optimized and designed to suppress the low frequency oscillation. Finally, based on the IADRC, the correctness and effectiveness of vehicle-side electrical quantity low frequency oscillation suppression when multi-CRH5 EMU pantographs are lifted and connected to traction network are verified by comparing PI control and ADRC control technology under Matlab/Simulink platform. This work is supported by National Key Research and Development Program of China (No. 2017YFB1201003-020), National Natural Science Foundation of China (No. 51767013 and No. 51867012), Natural Science Foundation of Gansu Province Education Department (No. 2017A-020), Science and Technology Research and Development Plan of China Railway Corporation (No. 2017J012-A), and Natural Science Foundation of Gansu Science and Technology Department (No. 18JR3RA111).