Robot-assisted　bonnet　polishing　systems　(RABPSs)　have　been　widely　studied　in　order　to　expand　the　application　of　bonnet　polishing　technology.　Currently,　low　working　stiffness　is　one　of　the　most　important　problems　that　needs　to　be　solved　for　RABPSs.　To　address　this　issue,　an　evaluation　method　for　the　working　stiffness　of　RABPSs　should　be　studied.　Therefore,　a　theoretical　and　experimental　study　is　presented　in　this　paper.　First,　the　static　stiffness　model　of　the　RABPS　was　built　based　on　the　Denavit?Hartenberg　(D?H)　method　and　the　compliance　matrix.　Subsequently,　the　force　characteristics　of　the　RABPS　during　the　bonnet　polishing　process　were　analyzed.　Next,　by　combining　the　above　static　stiffness　model　and　the　force　analysis　results,　a　new　index,　that　is,　the　normal　stiffness　coefficient　(NSC),　was　proposed　to　evaluate　the　working　stiffness　of　the　RABPS.　Finally,　the　performance　of　the　NSC　of　the　RABPS　was　verified　by　simulation　and　experiment.　Both　results　show　that　the　NSC　has　higher　accuracy　than　other　commonly　used　indexes　when　it　is　used　to　describe　the　working　stiffness　of　RABPSs.　This　research　provides　an　important　theoretical　foundation　for　the　optimization　of　the　working　stiffness　of　RABPSs　for　future　studies.