Objectives:Detailed element geochemical analysis are carried out on black shale of the Longmaxi Formation in the Yanyuan Basin ,southwest Sichuan province In order to investigate the formation of enrichment organic matter shale and expand the scope of shale gas around the Sichuan Basin.Methods:Basde on the field work,thruogh the microscope observation, the chemical analysis of main elements, trace elements and rare earth elements of rocks,the organic geochemical analysis such as TOC,Ro of Longmaxi Formation black shale. 〖WT5HZ〗Results:〖WTBZ〗The study shows that the black shale is rich in organic matter and higher siliceous contents， positive correlation of Al2O3 and TiO2 contents and negative correlated of SiO2 / Al2O3 ratios and Al2O3  contents show that the siliceous rocks with higher proportion of terrigenous argillaceous deposits. The high contents of Siexc,low contents of MgO, The elements Ba, As, Sb, and light rare earth enrichment ,positive Eu anomaly and microbial fossils in mineral and the presence of bentonite shows that siliceous rocks main causes for biogenic, affected by the hydrothermal sediment and volcanic deposit at the same time.Redoxsensitive elements V, Ni, U are enriched, NASC normalized REE contents of Ce negative anomalies, Eu positive anomalies and strawberry pyrite and some structural background discrimination diagrams show that they are formed in the reduced sedimentary water environment of the passive continental margin, and are affected by a certain degree of rising ocean currents or hot water and deposited the deepsea sediments at the same time .Conclusions:The biogenic and hydrothermal genesis (volcanic genesis) silicates are often rich in organic matter, and the reduced water environment is conducive to the preservation of organic matter. Therefore, the study area forming a widely distributed organic matter source rock of Longmaxi Formation and is the favorable development area of shale gas source rock.