This　paper　presents　a　novel　method　for　modeling　the　complexity　of　collaborative　design　systems　based　on　their　analysis　and　proposes　a　solution　for　reducing　complexity　and　improving　the　performance　of　such　systems.　The　interaction　and　interfacing　properties　among　many　components　of　a　complex　design　system　are　analyzed　from　different　viewpoints　and　then　a　complexity　model　for　collaborative　design　is　established　accordingly.　In　order　to　simplify　complexity　and　improve　performance　of　collaborative　design,　a　general　solution　of　decomposing　a　whole　system　into　sub-systems　and　using　a　unified　interface　mechanism　between　them　has　been　proposed.　This　proposed　solution　has　been　tested　with　a　case　study.　It　has　been　shown　that　the　proposed　solution　is　meaningful　and　practical.