Abstract: The tight oil reservoir of Middle Permian Lucaogou formation in Jimsar sag, Junggar basin has characteristics of “4-high” physical properties (i.e. relatively high density, high viscosity, high pour point and high wax content). The study on saturated hydrocarbon chromatography shows that the distribution of n-alkanes in the crude oil is complete and the crude oil never suffered from biodegradation; the maturity indexes of light hydrocarbons and sterane demonstrate that the oil is in the low maturity stage. The oil and source rocks in Lucaogou formation show the same thermal evolution stage and geochemical fingerprint characteristics, indicating a good affinity between them and tight-oil reservoir forming characteristics of “source and reservoir integration”. The oil generation simulation experiment shows that the accumulative hydrocarbon generation and expulsion quantities and oil discharge rate of the source rocks in Lucaogou formation at low mature stage are almost the same with those of normal source rocks at mature stage. Half-deep and deep lacustrine facies sediments, salinized reducing-strong reducing environments and good parent materials of hydrocarbons (Type Ⅰ-Ⅱ1 kerogens) are the main causes for large quantities of hydrocarbon generation and expulsion, and low-maturity tight oil accumulation of the good-quality source rocks in Lucaogou formation of Jimsar sag at the early stage.