In　this　paper,　a　new　tooth　modification　method　considering　the　contact　ratio　of　gears　and　a　new　method　for　calculating　the　mesh-in　impact　force　of　modified　helical　gears　are　proposed.　The　new　method　for　calculating　the　mesh-in　impact　force　is　based　on　tooth　contact　analysis　and　loaded　tooth　contact　analysis.　The　mesh-in　impact　position　can　be　calculated　accurately　via　the　new　method.　First,　the　procedures　for　creating　the　new　tooth　modification　and　the　details　of　calculation　method　of　the　mesh-in　impact　force　are　exhibited.　Second,　the　optimal　modification　of　the　tooth　flank　is　achieved　by　solving　the　optimization　problem.　Third,　a　dynamic　model　of　the　gear　system　considering　the　loaded　transmission　error　and　the　mesh-in　impact　force　is　used　to　study　the　dynamic　characteristics.　Ultimately,　numerical　examples　are　presented　and　the　simulation　results　suggest　that　the　amplitude　of　the　loaded　transmission　error　and　the　mesh-in　impact　force　can　be　reduced　more　effectively　based　on　the　introduced　new　tooth　modification　method.　And　the　mesh-in　impact　effects　should　not　be　neglected　in　gear　dynamic　analysis,　regardless　of　whether　the　tooth　modified　or　not,　especially　for　high-speed　gears.　©　IMechE　2020.