The　specimen　is　assumed　to　be　a　rigid　mass　in　the　design　of　the　shaking　table　control　system,　but　in　fact,　there　is　a　big　difference　between　the　specimen　and　the　shaking　table　system　caused　by　the　assumption　of　the　rigid　mass　of　the　specimen.　This　results　in　the　control　system　operating　in　a　narrow　frequency　band　and　poor　stability　reducing　the　replaying　precision　of　the　recorded　seismic　waves.　In　this　paper,　models　are　established　both　for　the　SDOF　and　MDOF　systems　which　are　designed　to　be　the　rigid　mass　and　non-rigid　mass,　and　then,　through　the　analysis　of　these　models,　the　performances　of　the　control　systems　are　evaluated.　The　conclusion　is　that　the　stability　of　the　system　designed　for　the　non-rigid　mass　performs　best　in　the　unloaded　condition　and　the　stability　of　shaking　table　is　greatly　improved.