Abstract: The petrological, physical and diagenetic characteristics and porosity evolution of the Jurassic clastic reservoirs are based on the examination of the representative core samples and thin sections, scanning electron photomicrographs, physical properties and mercury-injection data on the Jurassic clastic reservoirs in the Jiulongshan area, northern Qaidam Basin, Qinghai. The Jurassic clastic reservoirs consist mainly of coarse-grained sandstones, medium-grained sandstones and fine-grained sandstones. The cement types contain the carbonate cements, siliceous cements and authigenic clay minerals. The porosity types include the residual primary intergranular pores and solution openings, and the porosity and permeability are well correlated. The reservoir rocks have low compositional maturity and moderate to poor textural maturity, with the point-line contact types. At present, the reservoir rocks are tentative to be in Phase A of the anadiagenetic stages.