The invention relates to a novel selective water shutoff agent for oil well water shutoff, belonging to the technical field of tertiary oil recovery. The water shutoff agent is a composite system of gel particles with the mass concentration of 0.5-5% and a cross-linking agent with the mass concentration of 1-2%, which is prepared by using oilfield injection water; and the water shutoff agent is applicable to water plugging of an oil well at a temperature of 60-120 DEG C. The gel particles are formed by an acrylamide monomer or compounding the acrylamide monomer with a temperature resistant and salt-resistant auxiliary monomer and a cross-linking monomer through copolymerization; the cross-linking monomer is an ester monomer which has a double-bond structure and is easy to resolved when being heated, the expanding and resolving time of the cross-linking monomer at different temperatures can be controlled through adjusting the concentration of the cross-linking monomer, and bulk polymerization or inverse emulsion polymerization is adopted as a polymerization method. In the invention, basically no gel is formed on an oil layer, and high-strength gel can be formed on a water layer, thus defects in the prior art are overcome, the error shutoff phenomenon easily caused by the traditional polymer bulk gel in the oil layer is reduced to the greatest extent, and the capacity of an oil well suffering from water shutoff is ensured.