As　super-span　cross-rail　of　NC　heavy-duty　planer　type　milling　machine　tool　has　a　super　span　and　heavy　mass,　it　will　deform　seriously　after　installed.　Machining　an　arch　camber　on　X　direction　sideway　in　advance　is　used　to　compensate　for　influence　caused　by　its　deformation.　Finite-element　method　is　used　to　analysis　deformation　of　super-span　cross-rail　when　working.　Multinomial　is　used　to　fit　deformation　curve　based　on　deformation　of　the　X　direction　sideway　of　cross-rail　when　bottom-slide　and　ram　at　each　position.　At　the　same　time　this　paper　takes　into　consideration　of　the　impact　on　camber　curve　as　the　bottom-slide＇s　span　brings　about　2　deformation　curves.　A　method　based　on　2　deformation　curves　is　put　forward　to　design　and　optimize　the　camber　curve.　By　comparing　various　camber　curves＇　effect　on　the　deformation　of　milling　head,　this　article　proves　that　the　optimization　method　can　effectively　reduce　the　influence　of　cross-rail＇s　deformation　on　machine　tool＇s　accuracy.　©　(2013)　Trans　Tech　Publications,　Switzerland.