Real-time　dynamic　substructure　(RTDS)　system　consists　of　the　numerical　substructure　and　the　experimental　substructure.　The　two　parts　transmit　data　at　the　same　time　so　that　it　can　simulate　real　seismic　responses.　The　stability　of　RTDS　is　the　basis　of　the　experiment　s　feasibility　and　it　plays　a　key　role　in　the　implementation　of　a　test.　The　purpose　of　substructure　testing　is　to　experiment　large　size　specimens　which　are　nonlinear　based　on　the　existing　equipment.　However,　existing　researches　about　the　stability　of　RTDS　system　almost　concentrate　on　the　linear　stage　and　less　talk　about　the　effects　when　structures　get　into　the　nonlinear　part.　Therefore,　this　paper　focuses　on　the　effect　of　stability　of　nonlinear　structures.　A　novel　analysis　method　of　stability　was　developed　by　using　the　concept　of　＇gain　margin＇.　It　can　consider　the　loading　system　and　specimens　dynamic　characteristics　and　coupling　effects　at　one　time.　The　whole　process　of　stability　analysis　in　RTDS　testing　was　based　on　linear　system.　It　discussed　the　results　based　on　the　loading　system　of　shaking　table　and　the　actuator　respectively.　Results　show　that　the　stability　based　on　the　loading　system　of　actuator　can　be　determined　by　the　initial　stability　when　it　has　lower　initial　frequency.　Besides,　when　the　initial　frequency　is　higher　the　nonlinear　influence　should　take　into　account.　The　frequency　ratio　of　physical　and　numerical　substructure　is　the　main　factor　affecting　the　stability　of　RTDS　system　which　is　based　on　the　loading　system　of　shaking　table.　When　the　ratio　is　small　the　influence　of　the　nonlinear　of　the　stability　is　small.　And　when　the　ratio　is　large,　the　nonlinear　will　reduce　the　stability　of　the　test　system.　©　2017,　Editorial　Office　of　Journal　of　Xi＇an　University　of　Architecture　&　Technology.　All　right　reserved.