Single　degree-of-freedom　(DOF)　planar　linkage　mechanism　has　been　widely　used　in　bio-inspired　robots　because　of　its　simple　control.　However,　designing　a　single　DOF　mechanism　to　meet　the　complex　motion　law　of　creatures　is　difficult,　and　the　multiple　variables　of　six-or　eight-bar　mechanism　further　increase　the　design　difficulty.　A　design　method　of　single　DOF　planar　linkage　bionic　mechanism　is　proposed　in　this　paper.　Through　the　continuous　constraints　on　the　position　of　all　reference　points　with　constraint　descent　method,　the　requirements　of　the　bionic　mechanism　in　shape,　motion　trajectory,　and　posture　are　met.　As　for　the　mechanism　by　which　reference　point　coordinates　cannot　be　expressed　as　the　analytical　solution　of　the　driving　angle,　a　single　DOF　mechanism　is　equivalent　to　a　double　DOF　mechanism　containing　additional　optimization　variables,　which　further　improves　the　design　efficiency.　For　example,　the　design　process　of　six-and　eight-bar　bionic　mechanisms　is　given,　confirming　the　feasibility　of　the　method.　Then,　the　effects　of　the　constraint　conditions　on　the　design　process　are　analyzed　in　detail.　This　study　provides　a　reference　for　the　design　of　bio-inspired　robots.