A　whole　machine　tool　structure　is　a　muti-DOF　system,　it　is　usually　composed　of　many　parts　which　are　connected　with　joint　interfaces.　The　structure　is　complicated　that　the　accuracy　of　modal　test　is　always　in　a　low　level.　When　doing　modal　test,　equivalent　single　DOF　method　can　not　only　improve　the　efficiency　and　decrease　the　difficulty,　but　also　exclude　interferences　such　as　random　noise.　In　this　paper,　the　author　simplified　a　two-DOF　system　into　an　equivalent　single　DOF　system　and　derived　its　frequency　response　function,　then　analyzed　a　testing　device　for　fixed　joint　by　using　the　equivalent　single　DOF　method,　and　measured　several　groups　of　unit　area　dynamic　characteristic　parameters　of　fixed　joint　interfaces　in　different　materials　and　different　working　conditions.　The　dynamic　characteristic　parameters　can　be　used　in　further　study　of　machine　tool　joint　interfaces.