Optimal　tolerance　allocation　(OTA)　is　one　of　the　most　effective　ways　to　improve　the　machining　precision　of　machine　tools.　However,　the　existing　OTA　methods　have　the　problem　that　tolerance-cost　models　are　particularly　vulnerable　to　subjective　factors,　which　lead　to　tolerance　allocation　scheme　with　poor　applicability　and　reliability.　For　this　reason,　a　novel　OTA　method　that　simultaneously　considers　volumetric　error　and　processing　simplicity　index　(PSI)　is　proposed　in　this　paper.　Based　on　the　multi-body　system　theory　and　homogeneous　transform　matrix,　the　volumetric　error　prediction　model　based　on　tolerance　of　machine　tools＇　key　parts　is　firstly　established　by　taking　the　geometric　error　as　a　bridge　between　tolerance　and　volumetric　error　of　machine　tools.　Then,　the　weight　coefficient　of　each　tolerance　parameter　is　calculated　based　on　sensitivity　analysis　method.　Moreover,　PSI,　which　denotes　the　degree　of　processing　simplicity　to　reach　the　required　tolerance　value,　is　innovatively　proposed　to　replace　manufacturing　cost.　And　the　tolerance-PSI　model　is　established　to　replace　tolerance-cost　model.　Then,　an　OTA　model　is　established　by　minimizing　volumetric　error　and　maximizing　PSI.　The　NSGA-II　is　employed　to　obtain　OTA　scheme.　Finally,　simulations　and　experiments　are　conducted　to　validate　the　feasibility　and　effectiveness　of　the　presented　method.