This　paper　presented　a　sensitivity　analysis　method　for　tracing　the　volumetric　errors　of　a　machine　tool　that　made　the　greatest　impact　on　machining　errors　based　on　Monte　Carlo　simulation　theory.　A　machining　error　model　of　the　five-axis　gantry　machine　was　established　by　means　of　a　multi-body　system　theory　and　a　mathematical　model　of　S-shaped　test　piece　was　also　established.　The　mapping　relationship　between　the　cutting　points　and　tool　paths　was　deduced.　The　mathematical　model　of　cutting　surface　was　then　found.　According　to　the　principle　of　Monte　Carlo　simulation,　a　global　sensitivity　analysis　was　performed　on　the　machining　error　model.　Then,　the　geometric　error　parameters　that　make　a　great　impact　on　machining　errors　were　obtained.　By　sorting　the　sensitivity　analysis　results,　five　geometric　errors　δz(x),　δz(y),　δz(z),　δz(c),　and　δz(b)　made　the　greatest　influence　on　the　machining　error　model　of　z-axis.　Experimental　results　show　that　when　working　together　with　the　machine,　the　five　key　error　parameters　make　an　average　influence　on　z-axis　machining　error　up　to　88.2%.　Thus,　the　five　error　parameters　make　a　greatest　influence　on　z-axis　machining　errors.　This　method　can　provide　theoretical　guidance　for　improving　the　machining　precision　and　design　precision　of　the　five-axis　machine　tool.　©　2017,　Editorial　Department　of　Journal　of　Beijing　University　of　Technology.　All　right　reserved.