For　the　spindle　vibration　during　working　process　of　the　SH50　electrical　discharge　machining　(EDM)　machine　tool,　the　modal　characteristics　of　the　spindle　combining　theoretical　analysis　with　experimental　verification　are　studied.　First,　the　finite　element　model　is　built　in　ANSYS　Workbench,　and　the　theoretical　modal　of　the　spindle　is　analyzed.　Further,　the　weak　links　in　the　spindle　are　found　out　during　working　process　by　testing　its　operating　deflection　shapes　(ODS).　Then,　based　on　the　effective　independence　method　and　modal　assurance　criterion　(MAC)　matrix,　the　sensor　placements　of　key　parts　are　optimized　before　modal　experiment.　After　that,　the　modal　and　ODS　tests　are　carried　out　on　the　spindle　head　according　to　the　result　of　sensor　placements　optimization.　At　last,　some　advices　on　spindle　vibration　reduction　are　put　forward　after　confirming　the　theoretical　model　by　the　experimental　results.　The　simulation　result　shows　that　adding　a　backboard　can　greatly　reduce　the　vibration　of　the　spindle.　©　2017,　Editorial　Department　of　JVMD.　All　right　reserved.