The　effective　velocity　of　the　reflected　wave　in　rock　mass　is　of　significance　to　the　detection　of　crustal　structure　and　the　geophysical　seismic　exploration.　In　this　paper,　the　modified　characteristic　method　was　introduced　to　solve　P-wave　reflection　in　rock　mass　with　different　wave　impedances　on　two　sides　of　the　joint.　Effective　velocity　was　defined　to　characterize　the　propagation　velocity　of　the　reflected　wave　in　jointed　rock　mass.　The　effects　of　incident　frequency,　joint　stiffness　and　wave　impedance　ratio　on　the　effective　velocity　were　discussed.　The　results　show　that　when　the　stress　wave　propagation　in　＂hard-to-soft＂　rock　mass,　the　effective　velocity　increases　firstly　and　then　decreases　as　the　incident　frequency　and　the　joint　stiffness　increase,　while　the　effective　velocity　always　decreases　as　the　wave　impedance　ratio　increases;　when　the　stress　wave　propagation　in　＂soft-to-hard＂　rock　mass,　the　effective　velocity　decreases　as　the　incident　frequency　increases,　increases　as　the　joint　stiffness　increases　and　decreases　as　the　wave　impedance　ratio　increases.　The　wave　impedance　ratio　has　an　important　influence　on　the　effective　velocity.　The　effective　velocity　without　considering　wave　impedance　ratio　is　smaller　than　that　of　stress　wave　propagation　in　＂soft-to-hard＂　rock　mass,　but　larger　than　that　of　stress　wave　propagation　in　＂hard-to-soft＂　rock　mass.