The　impact　vibration　caused　by　sudden　changes　in　the　external　load　of　hydraulic　actuators　reduces　the　service　life　of　hydraulic　components　and　limits　the　high-precision　applications　of　hydraulic　actuators.　Therefore,　to　improve　the　underdamped　characteristics　and　impact　resistance　of　valve-controlled　cylinder　systems,　a　semi-active　impact　resistance　control　of　hydraulic　actuator　based　on　magnetorheological　damper　(MRD)　is　proposed.　Firstly,　based　on　the　shear　force　characteristics　of　magnetorheological　fluid,　a　multi-stage　flow　channel　MRD　is　designed,　which　is　connected　in　series　with　the　hydraulic　actuator　to　form　a　hydraulic　damping　actuator　(HDA).　Meanwhile,　based　on　the　mechanical　experimental　data,　the　parameters　of　MRD　dynamic　model　are　identified　to　ensure　the　accuracy　of　MRD　output　force.　Secondly,　a　drop　weight　impact　model　is　established,　and　a　sliding　mode　controller　which　can　track　the　command　current　in　real　time　is　designed,　so　as　to　realize　the　resistance　force　coupling　between　the　MRD　and　the　hydraulic　actuator　for　improving　the　impact　resistance　control　effect.　Then　the　dynamic　performance　test　and　impact　resistance　simulation　program　are　established　to　verify　the　excellent　dynamic　performance　and　impact　resistance　effect　of　the　HDA　with　the　sliding　mode　control.　Finally,　the　drop　weight　impact　experimental　platform　is　built.　The　experimental　results　show　that　under　the　impact　with　a　height　of　200　mm,　the　dynamic　offset　of　the　HDA　is　only　-1.25　mm,　and　the　time　to　return　to　the　original　position　is　0.08　s,　which　validate　the　excellent　performance　of　the　proposed　scheme　in　improving　the　underdamped　characteristics　of　the　valve-controlled　cylinder　system　and　the　dynamic　response　performance　of　the　hydraulic　actuator.