Abstract: The flow field and pressure field of low-permeability reservoirs are changed by dynamic fractures, significantly affecting the swept volume of water flooding. Comprehensively using the data of imaging logging, core analysis, rock mechanics experiments, production performance, and dynamic monitoring, the paper analyzes the characteristics and controlling factors of dynamic fractures in four typical ultra-low permeability reservoirs in the Ordos Basin, including Jing’an, Ansai, Xifeng, and Huaqing. Based on their geneses, dynamic fractures can be classified into two types, i.e., being formed by exceeding the rock formation’s fracture pressure and activated by natural fractures. According to impacts on the water cut of oil wells, they can be divided into unidirectional and multidirectional dynamic fractures. The evolution of dynamic fractures undergoes four stages:rapid growth to a certain scale, a fixed length, and a maximum length, and contraction stage; the extension direction of fractures is primarily controlled by the current maximum horizontal principal stress direction. The development intensity of dynamic fractures is influenced by factors such as matrix permeability, natural fractures, rock brittleness index, as well as development technology policies. The lower matrix permeability, more developed natural fractures, higher brittleness index, and stronger water injection development technology policies are easier for the formation of dynamic fractures, and the transition from unidirectional to multidirectional fractures. Dynamic fractures significantly affect the water cut increase regularity of oil wells and the distribution of remaining oil.