Abstract: The main difficulty of water flooding oilfields in the middle and late stages is to treat the high water cut of horizontal wells. In X offshore oilfield, however, the treatment of high water cut is difficult and the cost is high. Water plugging technology based on intelligent separate-production string are characterized by low cost, convenient operation and low risk, so it is suitable for water plugging of horizontal wells in offshore oilfields. In this paper, the remaining oil potential of watered out wells in X oilfield and the technical practicability of water plugging were determined by analyzing completion mode, current water cut, cumulative oil production, reservoir heterogeneity and water breakthrough time. And finally 5 candidate wells were selected for water plugging test because of their potential of water plugging with intelligent separate-production strings. Then, the implementation results of water plugging in one well were analyzed. The results show that the intelligent separate-production string is remarkable in water searching & plugging of large sections, should be popularized and applied further. It provides the technical instruction for water control and oil production improvement of oilfields at the late stage of development.