In order to control excessive water production in oil wells in offshore oilfield，the properties including injectivity， plugging capability，selectivity and water flushing resistance of an in-situ gelation water shutoff system in offshore oilfield， composed of 1% functional microsphere +3% functional monomer +0.8% gel promoter，were evaluated，and a field pilot test was carried out. The property evaluation results showed that the gelation time of the in-situ gelation water shutoff system from strength A to strength I was as long as 134 h，which could meet the requirements of on-site implementation for gelation time. The injection pressure was only 0.28 MPa in the sandpack with a permeability of 1020×10^-3μm²，and decreased with the increase of permeability， indicating the good injectivity of the system. The system had good properties of selectivity，plugging and water erosion resistance. It could preferentially enter the high permeability channeling area. After gelation，the plugging rate of the high permeability channeling area was more than 90%，and the plugging rate was only reduced by about 5.0% after 100 PV continuously water injection. After 60 days of immersing in water at the temperature of 65 ℃，the strength loss rate was only 4.1%，indicating that the system had good property of water immersion resistance after gelation. The field pilot test results showed that the water plugging effect of the in-situ gelation water shutoff system was obvious，the complex water channeling of the target well could be effectively controlled，and achieved the purpose of decreasing water production and increasing oil production of the target well.