A　flexible　manufacturing　system　(FMS)　improves　productivity　and　makes　it　more　efficient.　Maintaining　reliability　levels　and　reducing　costs　through　proper　maintenance　strategies　are　key　problems　for　the　development　and　application　of　a　FMS.　This　paper　proposes　a　grouping　preventive　maintenance　strategy　of　a　FMS　with　optimized　parameters　by　considering　both　reliability　and　cost.　In　this　work,　a　three-layer　evaluation　index　system　is　first　presented　to　accurately　estimate　the　reliability　of　the　FMS;　index　weights　of　each　layer　were　obtained　by　reliability　importance　modeling　and　analysis,　considering　maintenance　strategies.　An　element-grouping　preventive　strategy　is　proposed　based　on　an　influencing　analysis,　and　a　parameter　optimization　problem　(considering　reliability　and　maintenance　costs)　was　established.　In　this　strategy,　three　maintenance　methods　are　presented　for　the　elements,　including　low-level　maintenance　with　a　large　period,　low-level　maintenance　with　a　small　period,　as　well　as　the　combination　of　low-level　maintenance　with　a　small　period　and　high-level　maintenance　with　a　large　period;　the　effects　of　reliability　improvement　of　the　elements　on　the　subsystem＇s　reliability　were　analyzed　to　provide　evidence　for　element　grouping.　Finally,　the　proposed　method　was　applied　to　a　box-part　finishing　FMS;　the　results　indicate　that　this　method　can　effectively　reduce　maintenance　costs　on　the　premise　of　satisfying　the　reliability　requirements.