After long-term waterflooding development，anisotropic oil reservoirs often show uneven planar flooding and poor development effects. How to effectively adjust the well pattern is the key to redirection of waterflooding fluid flow，enhanced producing reserves，and balanced flooding development. The infill horizontal wells between diagonal oil wells with the inverted nine-point vertical well pattern are adopted，and the conformal transformation and the principle of mirror reflection of seepage mechanics are employed to create a seepage model of the injection-production pattern of diagonal infill horizontal wells. In addition，a method to evaluate the producing reserves capacity and effect of the injection-production well pattern is proposed. In light of the actual parameters of the oilfield，the law of producing reserves in the injection-production unit after horizontal well infill is analyzed in the reservoirs with anisotropic permeabilities. With regard to the effect of producing reserves and development conditions，the reasonable length and development system of diagonal infill horizontal wells between wells in anisotropic reservoirs have been determined. The results show that the horizontal well infill mode has good adaptability in the reservoirs with anisotropic permeabilities；the reasonable horizontal well infill length is about 0.5 times the diagonal interwell length of the original well pattern.