To　calculate　the　mesh-in　impact　force　of　modified　involute　helical　gears　more　accurately,　a　calculation　method　of　mesh-in　impact　force　for　modified　involute　helical　gears　is　proposed.　Based　on　the　tooth　contact　simulation,　the　＇tooth　surface-tooth　surface＇　contact　simulation　model　without　edge　contact　of　gears　and　the　＇tip　point-tooth　surface＇　contact　simulation　model　with　edge　contact　of　gears　are　set　up　in　consideration　of　the　base　pitch　difference　which　is　caused　by　the　tooth　surface　modification　and　loaded　tooth　deformation,　and　the　results　of　the　variation　of　mesh-in　impact　position　in　both　the　tooth　profile　and　the　longitudinal　directions　with　loads　are　calculated.　The　mesh-in　impact　force　is　calculated　according　to　the　energy　conservation　at　the　moment　of　teeth　coming　into　contact.　An　optimal　design　model　of　tooth　modification　is　set　up　based　on　the　proposed　method　and　the　genetic　optimization　algorithm.　Combined　with　an　example,　an　optimal　design　of　tooth　modification　with　minimum　mesh-in　impact　force　is　completed.　The　results　show　that　both　rotation　speed　and　load　have　great　effects　on　the　mesh-in　impact　force,　and　the　mesh-in　impact　force　of　the　modified　gears　is　reduced　effectively　within　the　set　load　range.　Rapid　increase　of　mesh-in　impact　force　may　be　caused　by　the　edge　contact　of　gears　and　should　be　avoided　in　the　design　of　tooth　modification.　©　2020,　Editorial　Office　of　Journal　of　Xi＇an　Jiaotong　University.　All　right　reserved.