Abstract: The characteristics of Ansai extra low permeability oil filed are the strong heterogeneity of reservoirs,the big starting pressure difference and driving pressure gradient,the natural fracture development and the declining productivity index after water break through.These factors are showing increasing negative effects on waterflooding in Ansai oil field.Based on the years of experiments that have been perfomed in Ansai oil field,the optimization of the water-injection parameters,the classification of fine water-injection units,the modification of water-in jection profile and the optimization of the in jection-production pattern were performed.Basic methods and main features of the exploration of Ansai Oil field were concluded; the timing of advancing waterflooding and well startup were proposed; the methods and features of enhanced water in jection and unstable waterflood ing was reached; the key mothod of triplicate fine waterflooding as a countermeasure for reservoir heterogeneity was brought forward; the practical application of section adjusting and well net pattern optimization was analyzed and evaluated.,The results may provide referential technical experience and guidance to future oil field exploration.