Suspended oil-drop in injected water can seriously plug the low permeability reservoir . It make water injected and oil produced hardly. In order to solve this problem, it need hardly to carry out the research on the laws of plugging to the low permeability reservoir caused by suspended oil-drop.The author has designed the core plugging experiment which used 3 kinds of size and 5 kinds of concentration of suspended oil-drop. The results show that: Throat diameter of core is 3.097μm and permeability of core is 3～4×10_-3μm₂. Permeability decreases and plugging occurs after suspended oil-drop injected. For the Ansai low permeable oil field, the concentration c and size d of suspended oil-drop are the important factors for plugging reservoir. When d≤3.16μm, the concentration plays a leading role for plugging reservoir compared with the size, part of suspended oil-drop can go through the core, the form is penetrable plugging; When d＞4.175μm, the size plays a leading role for plugging reservoir compared with the concentration, suspended oil-drop can"t go through the core, the form is shallow plugging; When 3.16μm＜d≤4.175μm, concentration and size play role for plugging reservoir together, the form is a transitional phase from penetrable plugging to shallow plugging. the author advice that the Ansai low permeable oil field should follow c ≤ 20.0 mg/L and d ≤ 4.175 μm of suspended oil-drop in injected water