Abstract: Shale oil in Mahu sag is an important exploration field in the Junggar Basin in recent years. Through core observation, rock slices, X-ray diffraction, scanning electron microscopy (SEM) and statistical analysis of physical and geochemical data, the reservoir characteristics and control factors of Permian Fengcheng Formation are studied. The analysis shows that the Permian Fengcheng Formation was formed in the saline lacustrine basin when the latter sedimented consistently, and two main sedimentary facies, fan delta and lake, were developed. The rock types are complex, and mainly in five types: terrigenous clastic rocks, dolomites and dolomitic rocks, organic rich shale, evaporites, volcanic rocks. There are many mineral components in the rocks, which are changed greatly in vertical direction, and searlesite, shortite, nahcolite and other uncommon alkaline minerals can be observed. The types of reservoir space are mainly vugs, intergranular pores, intercrystal pores and fractures, among which vugs and fractures are the favorable reservoir spaces, and are of dual medium structure of fractures-pores, which belong to micro-pore thin throat, low-porosity, low-permeability and ultra-low-permeability reservoirs. The control factors affecting the development of Fengcheng Formation shale oil reservoir mainly include the sedimentary environment and sedimentary facies, lithology, organic carbon content and the storage mode, alkaline mineral content, diagenesis and fractures, etc., among which sedimentary environment and sedimentary facies control the reservoir development scale and pore development in early stage; the fine siltstone, argillaceous siltstone and dolomite are of good reserve performance; organic carbon content was positively related with reservoir properties; the alkaline mineral content higher, the higher the reservoir porosity, dolomite diagenesis and dissolution decide the formation of secondary pores; under tectonic activities large quantities of fractures develop as reserve space and percolation channel, and this leads to the development of vugs around fractures, which plays a positive and constructive role in improving porosity, permeability and reservoir performance.