Droop　control　method　has　been　widely　applied　to　achieve　equal　power　sharing　among　distributed　generations　in　microgrids.　In　practice,　low-pass　filters　are　usually　required　to　mitigate　the　harmonics　and　noises　in　the　calculated　instantaneous　power　and　obtain　the　average　power　used　for　the　generation　of　frequency　and　voltage　references.　Unfortunately,　the　design　of　low-pass　filters　has　not　been　studied　systematically　in　the　existing　research.　This　paper,　by　proposing　a　small-signal　model　of　the　droop　controlled　system,　analyzes　the　effects　of　the　low-pass　filters　on　system　stability　and　provides　a　design　method　for　an　optimal　cutoff　frequency.　The　effectiveness　of　the　proposed　design　method　is　verified　by　simulations.