The　volumetric　error　coupled　by　geometric　errors　of　parts　is　the　main　reason　affecting　the　machining　accuracy.　How　to　determine　the　influence　degree　on　the　processing　precision　generated　by　the　geometric　errors　of　parts,　thus　distribute　the　geometric　errors　of　parts　economically　and　reasonably,　is　currently　a　difficult　problem　of　machine　tool　design.　Based　on　the　theory　of　multi-body　system　and　sensitivity　analysis,　a　new　method　of　identifying　the　key　geometric　error　sources　parameters　is　proposed.　Taking　a　four-axis　precision　horizontal　machining　center　as　example,　the　precision　model　of　the　machine　center　is　built　up　with　the　theory　of　multi-body　system,　thus　a　mathematical　model　for　error　sensitivity　analysis　of　four-axis　computer　numerical　control　machine　tools　is　established　with　matrix　differential　method,　finally　the　key　geometric　error　sources　which　affect　the　machining　precision　are　identified　after　sensitivity　coefficient　of　error　are　calculated　and　analyzed.　Calculation　and　example　show　that　geometric　error　factors　of　major　parts　that　have　relatively　significant　influence　on　comprehensive　spatial　error　of　the　machine　tools　can　be　identified　effectively,　thus　important　theoretical　basis　is　provided　for　improving　precision　of　machine　tools　reasonably　and　economically.　©　2012　Journal　of　Mechanical　Engineering.