The invention relates to a water shutoff agent for an extra-low permeability oil reservoir, and belongs to the technical field of oilfield development. The water shutoff agent is prepared by mixing an oil displacement agent, primary agents, a curing agent and water, wherein the oil displacement agent is petroleum sulfonate, fatty alcohol polyoxyethylene ether sulfate or a non-ionic surfactant; the primary agents are phenol and formaldehyde; and the curing agent is carboxylic acid, sulfonic acid or ammonium chloride. The invention has the beneficial effects that: the water shutoff agent has the initial state of aqueous solution and the initial viscosity of 3 to 10 mpa.s, and can be smoothly injected into the extra-low permeability oil reservoir; after the water shutoff agent is cured, a strong-strength solid matter is generated and closely and impermeably bonded with rocks, and has the water-phase breakthrough pressure of more than or equal to 35MPa, resistance to the temperature of 150 DEG C, the mineralization resistance of 25*10<4>mg/l, strong base resistance and strong acid resistance; and the requirements of permeability of less than 1*10<-3> mu m<2> and pore throat radius R of less than 8 mu m on the water shutoff of the extra-low permeability oil reservoir can be met.