The　hydrogen　Wankel　rotary　engine　(WRE)　is　an　ideal　power　plant,　which　can　meet　the　requirement　of　zero　carbon　emission　and　have　a　highly　compatible　between　the　high　flame　speed　of　fuel　and　the　good　high-speed　performance　of　engine.　However,　the　gas　leakage　is　one　of　the　key　problems　restricting　the　development　of　HWRE,　especially　the　leakage　at　the　spark　plug　cavity　is　the　most　serious.　In　this　work,　the　effect　of　leading　spark　plug　(LSP)　cavity　leakage　on　the　flow　and　combustion　performances　of　HWRE　is　investigated　by　the　numerical　simulation.　The　results　show　that　with　the　decrease　of　LSP　diameter,　the　values　of　fresh　mixture　leakage　(FML)　and　residual　gas　leakage　(RGL)　are　significantly　reduced.　The　reduction　of　FML　contributes　to　the　improvement　of　engine　economy,　and　the　proper　RGL　helps　to　strengthen　the　flow　field　and　the　turbulent　combustion　process.　Therefore,　at　the　original　LSP　location,　the　indicated　work　and　indicated　thermal　efficiency　first　increase　and　then　decrease　as　the　LSP　diameter　decreases.　When　the　diameter　is　reduced　to　9　mm,　the　indicated　work　and　indicated　thermal　efficiency　are　the　highest,　which　are　380.19　J　and　38.38　%,　respectively.　In　addition,　when　the　LSP　is　located　6.5　mm　to　minor　axis,　which　is　the　optimized　position,　although　the　reduction　of　LSP　diameter　significantly　reduces　the　FML,　the　higher　RGL　results　in　the　decrease　of　indicated　thermal　efficiency.　This　work　can　provide　the　design　scheme　and　theoretical　support　for　the　application　of　HWRE　in　the　field　of　transportation,　marine　and　aerospace.　©　2025　Hydrogen　Energy　Publications　LLC