Abstract: In separate-layer water blocking operation in deep reservoirs of Weicheng Oilfield, the characteristics of high reservoir pressure, low porosity and low permeability need a high performance of inorganic particle plugging agent. Conventional water plugging agent of inorganic particles together with ultrafine oil well cement of Grade G may cause such problems as high pressure, low strength and short effective period. In line with these problems and in combination with drawbacks in water plugging technique with ultrafine cement, research was conducted on the water blocking system technology. The ultrafine Grade G oilwell cement, ultrafine fly ash, active mineral powder and silica powder were mixed in different proportions to prepare a series of water plugging agents. Lab experiments were conducted on the rheology, anti-shrinking property and compressive strength of the water plugging agents, and it was determined that the formula of ultrafine powder plugging agent was 87.5% ultrafine cement + 7.0% ultrafine fly ash + 1.5% active mineral powder + 2.5% ultrafine silica powder + 1.5% retarder. This agent was successfully used in 13 wells to high pressure reservoir with low permeability in Weicheng Oilfield and addressed the technical difficulty in water plugging in this type of reservoirs.