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.　©　2018,　Engineering　Mechanics　Press.　All　right　reserved.