Through the reservoir description on Chang 4+5 reservoir group of the Upper Triassic Yanchang Formation in the Jiyuan oil field, Ordos Basin, it is pointed out that micro crack is of great importance to increase the permeability and the development level of superlow permeability reservoir. The research shows that the density of the micro crack is 786 strips/cm, its width is 3.70μm, its space is 1.2723mm, its porosity is 0.29%, its permeability is 3.32×10-3μm2. The storage ability of micro crack is 2.74% to that of matrix, and its permeability ability is 12.15 multiple to that of matrix. The existence of relatively high permeability reservoir lies in the difference and heterogeneity of the distribution of micro crack in superlow permeability reservoir. The growth and the distribution of micro crack resulted from the mutual actions between multiple periods palaeotectonic stress fields and the present based on microfacies belt and basement fault. It's possible to improve the success ratio in the oil and gas exploration if the aim is focused on to find favorable sedimentary facies, diagenetic facies and microfracture ribbon.