For the management of fractured large pores in high-water-bearing reservoirs, the general approach is to use transfer dissection and sealing. Conventional regulators have a limited regulating radius and can only produce blocking in the near-well zone, which is not ideal. Deep dissection technology can expand the radius of action and substantially improve the blocking effect. The existing deep-dissection agent system has problems such as high cost and poor effect, which affect its large-scale application. In this paper, to address these problems, a gel-type dissection modifier cross-linking agent was synthesized and optimized in the laboratory using low-concentration polymer, and the factors affecting the final gel formation effect were experimentally studied. The final polymer concentration was chosen to be 1500 mg/L~3000 mg/L, the poly-crossing ratio was 30:1, the pH was controlled at 7−9, and the temperature was controlled at 30−60 °C; the rubber was formed with good shear resistance and thermal stability, and had good adaptability to the high-mineralization environment. The optimal injection concentration of water-expanded rubber particles for this system was confirmed to be 3000 mg/L. Cryo-electron microscopy was used to observe the morphology of polymer gel formation and the adhesion of nucleated water-expanded particles to the gel, to clarify the mechanism of enhanced retention and sealing of nucleated water-expanded rubber particles by the multiple complex gel system, and finally to verify the sealing performance of the composite sealing system and determine the use effect by indoor simulation experiments with a two-dimensional flat plate model. This study is of great significance for the efficient development of high-water-bearing late reservoirs and further improvement of crude oil recovery.