A　coupled　mechanism　is　different　from　common　parallel　mechanism　with　independent　limbs,　so　it　is　impossible　to　analyze　the　constraint　and　mobility　by　the　traditional　method　based　on　screw　theory.　To　overcome　this　problem,　this　paper　proposes　a　new　procedure　to　determine　the　overconstraint　for　coupled　mechanisms　based　on　screw　theory,　where　a　coupled　mechanism　is　decomposed　into　several　basic　elements.　The　validity　of　the　new　procedure　is　further　proved　from　the　constraint　and　mobility　analysis　for　a　classical　mechanism.