Horizontal well productivity in bottom-water reservoir is an important content in the research of reservoir engineering analysis. The existing productivity formula for the bottom-water reservoir are based on short horizontal well flow model，which is not applicable to long horizontal well productivity calculation. In order to evaluate long horizontal well productivity more accurately，productivity formula for long horizontal well in bottom-water reservoir was deduced by using equivalent filtrational resistance，potential superposition principle and mirror image method considering both anisotropy and height of water avoidance. The case study shows that the anisotropy has a significant effect on the long horizontal well productivity. The production predicted by the new formula decreased by 53.52% compared with that obtained from the formula that neglects the effect of anisotropy. Oil production will be exaggerated when using short horizontal well productivity model to predict the long horizontal well. The maximum value can reach 5.01 times that of actual production. Calculation result of long horizontal well productivity model proposed in this paper for the bottom-water reservoir is more accurate than that before.