Aiming at the problems of uncertain influencing factors and low displacement ratio in the horizontal wells’ development in reservoirs with ultra-low permeability, the Yanchang Oilfield’s horizontal wells are studied in this article’s research of the principal control factors for water flooding development of horizontal wells. Using numerical simulation methods, the effects of different factors such as reservoir physical properties, sand bodies, injection-production well pattern parameters, fracture network parameters, and permeability of reservoir on the waterflooding efficiency of horizontal wells are analysed. Several factors impacting water flooding in horizontal wells in continuous and large-scale development sand bodies are adjusted using two parameters: daily oil output and cumulative oil production. According to the findings, the effectiveness of the injection-production system and inter-well displacement are influenced by the distribution of reservoir fractures and the connectedness of the fracture network. Imperfect well pattern leads to uneven water displacement effect, insufficient energy supplement in the later stage, and poor productivity of oil wells. When the horizontal section of the fracture network is 400 m long, in a spindle shape, with a fracture spacing of 100 m and a fracture radius of medium length, the displacement effect, control area, and water content rise are good. The small injection-production unit has fulfilled the anticipated development goals by optimizing the injection-production interval. The five-point well pattern’s huge sand body reservoir works well with its small injection-production unit. The study’s conclusions provide theoretical information for the technical and engineering tasks involved in the development of a horizontal well for water injection.