Abstract: Multiple rounds of huff and puff in tight reservoirs usually lead to a rapid decrease in production. Taking the M block of a tight reservoir with developed natural fractures as an example, a numerical simulation was conducted based on the physical properties of the matrix, natural fractures, and fractured fractures as well as the pressure difference to analyze the stress field of fracture tips and the features of fracture propagation. On this basis, a comparative analysis was then carried out to evaluate the development effect of water-injection huff and puff and unstable cyclic water injection. Results show that the formation pressure would grow with an increase in water injection time, and when the formation pressure was higher than the opening pressure of fractures, a complex and dynamic fracture network was formed with the expansion of natural fractures and the communication of fractured fractures. Unstable water-flooding can give full play to imbibition and displacement, and the change of water injection volume can effectively avoid water channeling and form relatively uniform flooding front. In addition, simulation results show that a significant increase of 18% in cumulative oil production of reservoirs with cyclic water injection compared with water-injection huff and puff. Therefore, transforming the development method into unstable water-flooding can effectively improve the oil production of horizontal wells in fractured tight reservoirs, providing a theoretical reference for optimizing the development of horizontal wells in tight reservoirs.