In　order　to　achieve　the　goal　that　the　tooth　modification　design　is　effective　under　the　most　commonly　used　load　and　other　loads　of　gear　transmission　system,　a　tooth　modification　design　method　of　vibration　reduction　for　involute　helical　gear　considering　the　measured　load　spectrum　was　presented,　and　its　validity　was　verified.　Firstly,　it＇s　necessary　to　obtain　the　load　spectrum　of　gear　transmission　system　before　tooth　modification　design,　and　then　the　working　load　range,　the　most　commonly　used　load　and　the　probability　of　load　occurrence　of　gear　transmission　system　were　obtained　through　load　spectrum　analysis.　Secondly,　the　equivalent　load　was　designed　based　on　a　series　of　loads　and　the　corresponding　load　occurrence　probability.　Thirdly,　equivalent　load　was　set　as　the　design　load　for　tooth　modification　design　as　the　input　parameter　of　the　next　step.　Finally,　two　tooth　modification　design　methods　including　improved　method　and　traditional　method　were　given.　Combined　with　an　example,　an　optimal　design　of　tooth　modification　with　the　minimum　amplitude　of　loaded　transmission　error　and　the　mesh-in　impact　force　was　completed　based　on　the　genetic　optimization　algorithm　to　verify　the　validity　of　the　equivalent　load.　The　results　show　that　the　amplitude　of　loaded　transmission　error　and　the　mesh-in　impact　force　are　effectively　reduced　after　optimum　tooth　modification.　The　good　transmission　quality　of　modified　gears　can　be　ensured　by　the　optimal　tooth　modification　design　under　the　designed　equivalent　load　within　the　most　working　loads,　especially　in　common　working　loads.　Under　the　equivalent　load,　the　most　commonly　used　load,　and　the　most　loads　in　the　working　load　range,　the　improved　tooth　modification　design　can　reduce　vibration　and　noise　more　effectively　than　the　traditional　modification　design.　©　2021,　Central　South　University　Press.　All　right　reserved.