Abstract: Fractured tight oil reservoirs exhibit dual-porosity characteristics of matrix and fractures, where severe water channeling during waterflooding makes it difficult to mobilize the crude oil in the matrix. In addressing water channeling, 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 expanded thermosetting composite resin plugging agent was developed, with a thermosetting base component as the core, combined with a regulator, toughening agent, and viscosifier. Performance evaluation results show that the plugging agent has an initial viscosity of 10–100 mPa·s at 25°C and a curing time of 20–40 hours at 65°C. When the mass fraction of the base component exceeds 15%, the system exhibits high compressive strength and stability, and a base component mass fraction greater than 15% is recommended for field application. Furthermore, in thin-tube breakthrough pressure tests, the plugging agent achieves a breakthrough pressure gradient of 5–10 MPa/m in tubes with diameters of 3–10 mm. The low initial viscosity and high plugging strength of this agent resolve the conflict between injectivity and plugging performance, offering a new technical approach for efficient water shutoff in fractured tight oil reservoirs.