According to the plugging difficulty in fractured reservoir of Wangyao infilled block, the urea-formaldehyde resin plugging agent with high strength was synthesized by formaldehyde and carbamide at 55℃, and its performance was studied. The results showed that the dissolution rate in injected water, paraffin and mixed acid (10% HCl＋3% HF) was 1.54%, 2.40% and 14.30%, respectively. Its hydrotrope, oil and acid soluble were all lower which indicated good stability. With increasing temperature, the curing time of urea-formaldehyde resin shortened, and its compressive resistance increased fast. The curing time was 10 h, and the compressive resistance was 12.2 MPa at 55℃. With increasing degree of mineralization, the compressive resistance decreased and the curing time extended. The curing time was 12 h, and the compressive resistance was 4 MPa when the degree of mineralization was 50 g/L. The plugging efficiency in fractured reservoir was higher than 95%, and the breakthrough pressure was more than 24 MPa which showed high plugging off intensity. The "weak gel and urea-formaldehyde resin" system was applied for field construction in converted injector well Wang19-06. Cumulative quantity of oil was 545.56 t, water cut decrease was 475.92 m3, and valid period was more than 10 months. The pilot effects of increasing oil production and decreasing water cut were good.