Abstract: Fractured tight oil reservoirs exhibit dual-porosity characteristics of matrix and fractures, and fracture water channeling during waterflooding makes it difficult to mobilize the crude oil in the matrix. For water channel control, the plugging agent is required to possess both a high breakthrough pressure gradient to initiate matrix oil flow and good injectivity to prevent fracture reopening. To this end, a novel swelling thermosetting composite resin plugging agent was developed with a thermosetting main agent as the core, combined with a regulator, a toughening agent, and a viscosifier. The performance evaluation results indicate that the plugging agent has an initial viscosity of 10–100 mPa·s at 25 ℃ and a curing time of 20–40 h at 65 ℃. When the mass fraction of the main agent is greater than 15%, the system exhibits high compressive strength and stability, and a main agent mass fraction of greater than 15% is recommended for field application. The results of capillary breakthrough pressure tests indicate that the plugging agent achieves a breakthrough pressure gradient of 5–10 MPa/m in thin tubes with diameters of 3–10 mm. The research indicates that the low initial viscosity and high plugging strength of this agent resolve the conflict between injectivity and plugging performance, providing a new technical approach for efficient water shutoff in fractured tight oil reservoirs.