The integrated oil reservoirs have stepped into extra-high water cut stage. Original technologies don’t fit to the current development situation，and the design of horizontal wells has become more difficult and the development effect is restricted to the deficient understanding about oil-displacement mechanism of horizontal wells at this stage. A large-scale consolidated model was built based on the similarity criterion in physical simulation according to the practical production data. Enhancing the oil recovery by the horizontal wells was studied. EOR mechanism of the horizontal wells can be re? vealed through the introduction of contribution rate of oil displacement efficiency and contribution rate of sweep efficiency.The influence of vertical permeability ratio，interlayer permeability，horizontal wells direction and the drilling time on oil recovery were studied with numerical simulation. The result shows that sweep efficiency may be improved at early stage of production and the oil displacement efficiency should be enhanced in the extra-high water cut stage for the horizontal wells.The horizontal well is more perpendicular to the line of injection wells the higher the vertical permeability ratio is and the lower the interlayer permeability is. The contribution rate of oil displace efficiency is higher at the same water cut the earlier the deploying time of horizontal well is and the lower the contribution rate of sweep efficiency is.