As　a　basic　osteotomy　technique,　U　osteotomy　can　be　applied　for　certain　complex　foot　and　ankle　deformities.　Gradual　correction　cases　using　Ilizarov　apparatus　and　Taylor　Spatial　Frame　have　been　reported.　This　paper　proposes　a　novel　parallel　distraction　apparatus　for　U　osteotomy　(PDA-Uos)　to　supplement　the　correction　equipment　for　surgeon.　Designed　with　an　adjustable　structure,　the　PDA-Uos　can　adopt　different　assembly　shapes　(joint　connection　points).　However,　the　influence　of　the　change　in　assembly　shape　on　interference-free　workspace　and　self-structural　performance　of　the　external　fixator　have　received　little　attention.　To　address　this　issue　and　enhance　the　selection　of　the　most　suitable　assembly　shape　for　patient,　an　algorithm　to　obtain　the　interference-free　workspace　of　different　assembly　shapes　is　proposed　based　on　the　inverse　position　solution　of　the　PDA-Uos.　Additionally,　correction　ability　indices　are　defined　according　to　the　requirements　of　accurately　controlled　motion　and　high　structural　stiffness　of　the　external　fixator　along　the　correction　path.　The　results　of　simulation　cases　indicate　that　the　interference-free　workspace　and　the　correction　ability　vary　according　to　the　assembly　shape　and　thus　the　proposed　method　can　be　used　to　select　an　assembly　shape　with　sufficient　workspace　and　the　best　correction　ability　before　performing　correction　for　a　given　patient.　©　2020,　International　Federation　for　Medical　and　Biological　Engineering.