The pressure gradient between the injection and production wells is the core of displacement slug length optimization in the decision-making process of profile modification. However, displacement slug length calculation method is based on experience and lack of theoretical basis currently. In this article, seepage mechanics theory is used to calculate the interwell pressure gradient under the premise of the homogeneous reservoir. The influence of different injection-production well spacing on interwell pressure gradient was analyzed. The finite element numerical simulation was used to calculate the interwell pressure gradient under high permeable zone and profile modification. The influence of different geological conditions and development conditions on interwell pressure gradient was analyzed. Through this article’s demonstration, the permeability of high permeable zone has little influence on interwell pressure gradient. The profile control treatment has great influence on interwell pressure gradient and the plugging agent slug sustain major pressure drop between injection and production wells. An interval of less than 10 meters from the bottom of the well is the main pressure drop zone. It is recommended to inject a 10 meters displacement slug after injecting the major gels.