Optimization　is　applied　in　developing　of　double　public　gear　speed　change　system　of　main　transmission,　the　optimal　case　can　be　obtained　under　certain　condition,　which　make　the　total　center　distance　of　spindle　box　always　spreading　the　smallest.　This　paper　firstly　introduces　the　particularity　of　the　double　public　gear　transmission　and　deduces　the　constraint　condition　of　the　transmission　ratio　under　the　different　number　of　the　spindle　speeds.　Then　how　to　establish　optimization　design　model　is　stated　in　detail.　Finally,　the　accuracy　of　the　model　is　proved　by　an　example.　The　results　show　that　the　system　is　correct.　It　is　characterized　by　the　pleasing　interface,　convenient　operation　and　better　understanding.　©　Springer-Verlag　Berlin　Heidelberg　2012.