During the developing of oil and gas field with horizontal wells, there always exists a damage zone with low or ultra-low permeability near the wellbore. Through acidizing, the permeability near the wellbore will become higher than ever before, eventually the well can get high productivity, so it is of great importance to accurately predict the productivity of horizontal wells after acidizing. Based on the seepage principle, this paper simplified the three dimensional seepage mode of horizontal wells into two two-dimensional seepage, and applied the area equivalence principle, conformal mapping method, complex potential theory and the law of equivalence percolation resistance to deduct a new productivity formula of horizontal wells after acidizing. Through case study and correlation, the results calculated by the new formula and classic productivity formula of horizontal wells are smaller than practical output, but the relative error from the results calculated by new formula is the smallest. So, the new formula is more practical for predicting the productivity of horizontal wells after acidizing. The study can provide new research method for the productivity prediction and dynamic analysis of horizontal wells after acidizing.