Prorocentrum　donghaiense　is　a　harmful-algal-bloom-forming　species　of　planktonic　dinoflagellates　widely　distributed　around　the　world,　which　threatens　the　marine　environment　and　human　health.　Bacteria　are　promising　biological　agents　to　control　algal　growth　in　HABs.　Previously,　we　isolated　an　Alteromonas　sp.　FDHY-03,　a　P.　donghaiense-lysing　bacteria　strain,　from　Xiapu　Sea　area　of　China.　In　order　to　improve　the　algicidal　activity　of　this　strain,　we　optimized　the　medium　composition　and　culture　conditions.　Based　on　single-factor　method　experiment　design,　the　optimum　medium　component　of　algicidal　effect　for　strain　FDHY-03　was　sucrose　and　peptone.　The　result　of　Plackett–Burman　design　indicated　that　three　significant　factors　(sucrose,　peptone,　and　rotational　speed)　appeared.　Finally,　the　concentrations　of　key　factors　were　confirmed　by　central　composite　design　(CCD)　and　response　surface　methodology　(RSM).　Under　the　optimized　medium,　the　algicidal　rate　of　strain　FDHY-03　against　P.　donghaiense　improved　by　67.15%,　and　the　OD600　value　increased　by　2.86-fold.　The　optimal　source　and　condition　were　sucrose　0.46%　(w/v),　peptone　4.25%　(w/v)　addition,　and　rotation　speed　255　rpm.　Overall,　this　study　provides　an　optimized　method　and　optimum　medium　for　improving　the　algicidal　activity　against　P.　donghaiense,　and　has　a　positive　influence　on　algae-lysing　bacteria　for　controlling　the　blooms　of　the　algae　in　the　environment.　©　2022　by　the　authors.