In　order　to　study　the　effect　of　initial　surface　roughness　on　water　droplet　erosion　resistance　of　last　stage　blade　substrate　of　steam　turbine,　eight　17-4PH　samples　were　grounded　and　velvet　polished　by　different　mesh　metallographic　sandpaper　to　establish　sample　with　different　initial　surface　roughness.　The　water　droplet　erosion　experiments　were　carried　out　in　the　high-speed　jet　water　erosion　experiment　system,　and　the　mass　and　micro-morphology　of　each　sample　were　measured　by　using　precision　electronic　balance　and　ultra-depth　of　field　microscope　respectively　at　each　experimental　stage,　and　the　measurement　of　water　erosion　trace　width　and　maximum　water　erosion　depth　were　also　completed　at　the　same　time.　On　the　basis　of　experiments,　LS-DYNA　was　used　for　numerical　simulation　to　verify　the　reliability　of　experimental　results　again.　Results　show　that　the　smoother　the　initial　surface　of　sample,　then　the　smaller　the　mass　loss,　the　stronger　its　water　erosion　resistance.　On　the　contrary,　the　rougher　the　initial　surface　of　sample,　the　more　severe　the　surface　irregularity,　the　more　times　the　water　droplets　concentrated　at　the　lowest　point　of　pit　when　water　droplets　flow　laterally　after　impact　is　completed,　thus　accelerating　the　formation　of　initial　crack　and　lateral　expansion,　the　poorer　the　water　erosion　resistance　of　sample.　At　same　water　erosion　time,　the　smoother　the　sample　surface,　the　later　the　complete　erosion　trace　appear,　the　narrower　the　water　erosion　trace　width.　However,　the　maximum　water　erosion　depth　of　sample　is　not　affected　by　the　initial　surface　roughness.　The　numerical　simulation　results　are　in　good　agreement　with　the　experimental　results.　Copyright　©　2020　ASME.