Abstract: Based on the data of cast thin section, SEM, Electronic probe, X-ray diffraction and petrophysical properties etc., the paper detailedly studies the petrologic features and diagenesis of the Middle-Lower Permian clastics in the northwestern margin of Junggar basin, based on which, the influence of the diagenesis in alkaline environment on accumulative properties of reservoirs is discussed. The results show that the diagenesis in alkaline environment is commonly noted in the Middle-Lower Permian strata, such as tephra hydrolysis, tephra albitization, formation of zeolites and chlorite and so on. Various associated pores including alkaline dissolved pore of tephra, albitized associated pore and intercrystalline micropore in clay minerals are developed in alkaline environment. The influences of diagenesis in alkaline environment on the Middle-Lower Permian clastic reservoirs mainly include strong impaction of reservoir fluids resulted from alkaline fluids, i.e., high reservoir compaction ratio in alkaline fluids, which is unfavorable for primary pore preservation; high authigenic mineral content of reservoir under the diagenesis in alkaline environment and local cementation with large pore reduction amounts; high pore increase efficiency due to the corrosion of alkaline fluid. Alkaline dissolved pore in tephra can be the important accumulation space