To　perform　precise　measurement　on　mega　gears,　the　principle　of　mega-gear　in-site　measurement　system　with　laser　tracker　and　corresponding　key　techniques　are　originally　introduced.　Considering　the　mega-gear　in-site　measurement　system　with　laser　tracker　integrates　the　large-scale　measurement　capacity　of　laser　tracker　and　the　high　accuracy　of　coordinate　measuring　machine,　the　laser　tracker　is　used　to　establish　the　coordinate　system　both　for　mega　gears　and　the　Three-coordinate　Measurement　Platform　(TMP),　thereby,　their　coordinate　systems　are　associated　by　the　coordinate　system　of　the　laser　tracker,　and　the　module　and　algorithm　of　coordinate　system　fitting　are　proposed　simultaneously.　Meanwhile,　a　novel　module　of　attitude　adjustment　of　TMP　is　established,　which　can　be　accomplished　by　the　system　of　attitude　adjustment　to　ensure　that　the　position　relationship　between　TMP　and　the　axis　of　gear　can　meet　the　measuring　requirements.　The　results　illustrate　that　the　principle　of　mega-gear　in-site　measurement　system　with　laser　tracker　is　correct　and　feasible,　and　in　consequence　it　can　meet　the　six　grade　precision　measurement　requirements　of　mega　gears.