The invention discloses a W / O / W type active viscous oil blocking agent. The blocking agent comprises internal phase water, viscous oil and external phase water. A nonionic surfactant is adopted between the internal phase water and the viscous oil. An ionic surfactant is adopted between the viscous oil and the external phase water. According to the invention, a W / O / W composite structure is adopted, W / O with a relatively large viscosity is coated by the external phase water layer, such that defects in prior arts of high viscosity, high construction pressure and poor selectivity upon an oil-water layer during an active viscous oil blocking agent injection process are overcome. With the blocking agent provided by the invention, the ground fluidity is good, and the injection pressure is low. When the blocking agent enters the stratum, the emulsion is demulsified, and W / O emulsion with a high viscosity is released, such that oil well water exits can be effectively blocked, and defects of existing active viscous oil blocking agents are overcome. The technical scheme provided by the invention is especially suitable for technical features of offshore oilfields.