Polarization　effect　of　shear　wave　is　mainly　utilized　to　obtain　shear　wave　arrival　time　in　downhole　method　of　shear　wave　velocity　test.　However,　the　polarity　of　the　P-wave　of　received　signals　is　observed　to　be　reversed　in　in-situ　tests.　Therefore,　it　is　vital　to　study　the　theoretical　basis　that　the　P-wave　keeps　the　same　polarity　while　the　shear　wave　changes　polarity.　A　three-dimensional　finite　element　numerical　model　was　established　in　order　to　simulate　the　polarization　effect　of　the　downhole　method　excited　by　surface　forward　and　reversed　hammer　strike.　The　vibration　responses　under　surface　excitation　at　different　depths　were　analyzed　by　time-domain　lumped　mass　dynamic　finite　element　method　with　an　explicit　step-by-step　integration.　It　is　shown　that　both　S-waves　and　P-waves　are　clearly　observed　to　be　180　degrees　phase　difference　from　the　horizontal　signal　traces　with　the　direction　of　excitation　generated　by　reversed　impulse,　which　is　contrary　to　the　practical　engineering　cognition.　In　order　to　find　out　the　reason　behind　this　phenomenon,　numerical　simulations　of　three　possible　scenarios　(inclined　excitation,　geophone　deflection　and　geophone　inclination)　were　carried　out.　The　results　indicate　that　the　combined　influence　of　inclined　excitation　and　geophone　inclination　is　the　main　cause　that　leads　P-wave　to　keep　the　same　polarity　while　the　shear　wave　changes　polarity.　Furthermore,　a　method　based　on　the　time　interval　during　load　peak　and　response　peak　is　proposed　to　obtain　reliable　shear　wave　velocity,　which　is　verified　to　have　a　shear　wave　velocity　closer　to　preset　velocity　of　the　model　in　numerical　simulation　and　better　than　that　from　the　peak　to　peak　method　and　cross-correlation　method.　©　2020,　Science　Press.　All　right　reserved.