The hot issues of strong heterogeneity，complex distribution and difficult prediction of organic matter exist in shale reservoirs in Chang73 Member of Mesozoic Yanchang Formation in southeast Ordos Basin. Considering this，based on the data of drilling，logging，core，thin section and trace element analysis to explore the development characteristics of organic matter and its enrichment model in shale reservoirs of Chang73 Member. The results indicate that the semi-deep to deep lacustrine shale in Chang73 Member in the sedimentary period has great potential of shale gas，with a high abundance of organic matter，large thickness，and good continuity，and the most organic matter is Ⅱ1 type. Organic matter is widely developed in thick layered shale rocks and enriched along sedimentary laminae. Affected by the Indosinian movement，the basement subsides intensively during Chang73 Member sedimentary period，which forms a large-scale sedimentary center and expands the accommodation space of the lake basin. Moreover，a humid and semi-arid climate is favorable for nutrient minerals to enter the lake basin and promote aquatic organism accumulation，and volcanic events also lead to eutrophication of water bodies and improved organic matter productivity. A long-term and stable anoxic and reductive environment provides key conditions for the preservation of sedimentary organic matter.