Abstract: Gabbro is mostly partial melting of mantle rocks，the formation of mafic dykes and alkaline syenite are usually related with the extensional tectonics，the research on Triassic gabbro，mafic dykes and alkaline syenite in Hainan Island has important implications to its tectonic background. Systematic geochronology and geochemistry of gabbro and mafic dykes in Wanning and syenite in Fenjiezhou，Hainan Island，were done in this paper，and the tectonic significance was discussed. The result of LAICPMS zircon UPb dating shows that the age of gabbro and mafic dykes is about 240 Ma，the age of syntite is about 231 Ma. The major element shows that gabbro and mafic dykes belong to alkaline series and subalkaline series，syenite in Fenjiezhou belongs to typical alkaline rocks. The REE elements of gabbro and mafic dykes both have a feature of enrichment in LREE(LREE/HREE=722～85 and 811～1110)，the trace element has a feature of island arc，which is enrichment in LILE(K，Rb，Ba，Th)but lack HSFE(Nb，Ta，Zr，Hf)，while the trace element of syenite in Fenjiezhou shows a feature of Atype granite. The tectonic background of gabbro，mafic dykes in Wanning and stenite in Fenjiezhou，Hainan Island，is intracontinental extension environment，shows that Hainnan Island was in the stage of stress relaxation of Indoorogenic during 240～230 Ma.