Aiming at several problems in Jiyuan Oilfield of Ordos Basin, strong heterogeneity, complex micro pore structure, its effect on saturation of mobile fluid and so on, the writers conduct a series experiments including the techniques of physical properties, casting thin slice, the SEM, particle size, high pressure mercury penetratio and nuclear magnetic resonance. The research shows that the reservoir space can be divided into primary pore, secondary pore and microfracture. Based on the capillary pressure curve and its corresponding parameters, the writers divide the pore structures into four types. Different pore structure correspond to different reservoir space. Affected by its micro pore structure, the reservoir capacity and percolation ability of the reservoirs gets worse successively. The movable fluid saturation of NMR is similar to its parameters of pores structure. The throat characteristic and the connectivity between pore throats are the crucial factor that decide if physical quality is good in low permeability layer, besides permeability also have an effect on the saturation of movable fluid.