Chang 3 reservoir of Yanchang Formation in Weibei Oilfield is a low-ultra-low permeability reservoir. The relationship between pore throat microstructures and fluid availability and the percolation mechanism are unclear during the water injection development of this reservoir. With regard to this problem，microscopic water flooding experiments on real sandstones were performed to investigate the pore throat microstructures，water-flooding percolation characteristics，and their influencing factors in Chang 3 reservoir of Yanchang Formation in Weibei Oilfield，using the casting thin-section analysis，scanning electron microscopy，high-pressure mercury intrusion，physical property analysis，and other test methods.The results show that Chang 3 reservoir space in the study area is dominated by intergranular pores，intergranular dissolved pores，and intragranular dissolved pores，with flaky and necking throats. The pore throat structures can be divided into TypesⅠ，ⅡandⅢ，and their corresponding percolation characteristics in reservoirs and water displacement efficiencies are distinctly different. The water displacement in TypesⅠ-Ⅱpore throat structures mainly belongs to the piston-like，while that in TypeⅢpore throat structure is mainly the non-piston-like. In the pore networks，the water displacement pattern is mainly uniform and networked-uniform in TypeⅠpore throat structure；while networked in TypeⅡstructure；meanwhile finger-shaped–networked in TypeⅢstructure. In addition，the physical properties of the reservoirs corresponding to Types Ⅰ-Ⅲpore throat structures gradually deteriorate；the number of small pore throats increases；the ultimate oil displacement efficiency decreases successively. Reservoir physical properties，pore throat structure，displacement pressure，and water injection multiples all influence the water-flooding percolation characteristics and the ultimate displacement efficiency. The more favorable reservoir physical properties and the better pore throat structure can lead to the higher water flooding efficiency，and appropriately increasing injection pressure and water injection multiples can enhance the ultimate water displacement recovery.