To clarify the effect of previous well pattern infillings of Daqing Oilfield，this paper analyzed changes in the water-breakthrough thickness ratio，water-washing thickness ratio，and oil displacement efficiency of various oil layers on the basis of cored well data obtained 5-10 years after the adjustments. The result shows that the basic well pattern enabled the effective producing of oil layers with an effective thickness above 2.0 m，and the water-breakthrough thickness ratio reached 75.8%. The primary well pattern infilling significantly improved the water-breakthrough thickness ratio of oil layers with effective thicknesses of 2.0 m or less，and the water-breakthrough thickness ratio with effective thicknesses of 0.5-2.0 m increased by 195.0%~264.4%，thus the untabulated oil layers effectively produced well too. The secondary infilling further improved the water-breakthrough thickness ratio of oil layers with effective thicknesses between 0-1.0 m by nearly 50.0%. The tertiary infilling did not improve the water-breakthrough thickness ratio of the oil layers considerably，the water-breakthrough thickness ratio of the thin and poor oil layers increased no more than 2.3%，which represented the gradually worsening effect of well pattern infilling. The high interlayer heterogeneity, the large producing zone, and the decreasing interlayer water cut difference are key factors for the low water-breakthrough thickness ratio of the thin and poor oil layers and the worsening effect of previous infillings. According to the analysis above，a well pattern infilling mode that equally emphasizes the reduction of producing section length and well spacing was proposed.