Abstract: Combing the advantages of numerical simulation with experimental testing, real-time dynamic substructure (RTDS) testing provides a new experimental method for the investigation of engineering structures. The premise of a successful RTDS testing is to ensure its stability. However, most researchers focused on the stability of RTDS testing in terms of single degree of freedom systems. For multi-degree of freedom (MDOF) systems, the parameters required in the stability analysis are often complex and the index of stability assessment has no specific physical meaning. This work developed an analytical method for MDOF RTDS system incorporating the mode superposition method and the concept of gain margin. The effectiveness of this method was verified experimentally. Meanwhile, the influencing mechanism of time delay compensation on the stability of MDOF RTDS system was uncovered. The analytical and experimental results show that the developed method can accurately evaluate the stability of MDOF RTDS system. Delay compensation can be beneficial as well as detrimental to the stability of RTDS system.