A　three-dimensional　simulation　on　flow　behavior　and　lean　combustion　in　a　hydrogen　direct-injection　gasoline　rotary　engine　was　implemented.　The　combined　influence　of　hydrogen　injection　pressure　(HIP)　and　nozzle　diameter　(HND)　was　numerically　analyzed　in　the　critical　injection　state.　Results　showed　that　with　the　increments　of　HIP　and　HND,　further　penetration　and　larger　dispersion　angle　of　jet-flow　were　presented.　Increasing　HIP　led　to　larger　jet-flow　area　and　more　jet-wall　interaction,　as　well　as　greater　deflection　degree　of　hydrogen-flow　after　crashing　upon　the　rotor　surface.　High-speed　region　expanded,　while　more　jet-wall　impingement　occurred　with　the　increment　of　HND.　The　spark　ignited　earlier　and　flame　propagated　faster　as　the　HND　modified　smaller.　As　the　HIP　increased,　the　flame　speed　decreased　slightly.　The　difference　in　the　local　Phi　distribution　and　in-cylinder　velocity　field　were　the　intrinsic　mechanism　of　the　HND　impact,　and　the　variation　in　the　mixture　concentration　and　TKE　magnitude　were　the　root　causes　of　the　HIP　effect.　The　pressure　and　HRR　traces　were　shifted　to　the　downward　clearly,　while　the　peak　positions　were　forming　later　as　the　HND　extended　larger.　No　perceptible　difference　in　the　peak　positions　of　pressure　and　HRR　were　obtained　with　the　HIP　decrease,　although　pressure　tended　to　increase　along　with　heat　release　retard　to　be　shortened.　The　combustion　rate　decreased　with　increments　of　HND　and　HIP,　the　timings　of　the　initial　consumption　were　delayed,　and　a　proportional　correspondence　of　reactants　and　intermediates　was　witnessed.　The　reduced　NOx　formation　was　obtained　by　increments　of　HND　and　HIP,　and　an　inverse　correlation　can　be　seen　between　soot,　CO,　HC　generations　and　NOx　concentration.　The　scheme　of　0.4　Willa　HIP　and　1　nun　HND　was　the　optimum　configuration.　Compared　with　a　larger　HND　and　higher　HIP　configuration,　the　peak　pressure　and　maximum　HRR　were　increased　by　24.1%　and　40.5%,　and　the　productions　of　soot,　CO,　and　HC　were　reduced　by　55.7%,　61.5%,　and　54.7%,　respectively,　though　the　NOx　generation　was　increased　to　some　extent.　It　was　recommended　in　practice　operations　that　the　combination　of　lower　HIP　and　smaller　HND　left　a　positive　influence　on　improving　the　combustion　characteristics　of　Wankel　engines.