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The most popular oil displacement technique in oilfield development is water flooding, yet sustained water flooding makes it simple to create water channeling in the formation, lowering development efficiency. A high strength agar was synthesized by using raw agar and 3-chloro-2-hydroxy-propyl trimethyl ammonium chloride, and it was then combined with PAM to prepare a plugging system to address the issue of water channeling in old oilfields. The successful synthesis of a modified agar was demonstrated by FITR, EM, TG/DSC, and its temperature resistance and agar strength were improved. Gelation and flow experiments were used to test the gelation and plugging characteristics of the plugging system. The results show that the plugging system is a pseudoplastic fluid, the temperature resistance less than 120 °C, the salinity resistance less than 1 × 105 mg/L, the gelation time of is 1 ∼ 3d, the gelation strength reaches grade I, the resistance coefficient less than 10, the breakthrough pressure gradient greater than 20 MPa·m−1, the plugging ratio greater than 90%, and the oil recovery ratio increases 22.8%. This work's study has a significant impact on enhancing oil recovery. [ABSTRACT FROM AUTHOR]