The three-dimensional anisotropy of actual reservoirs and the typical linear flow characteristics in horizontal well seepage are not considered in commonly used horizontal well productivity prediction formula，and thus the productivity prediction results tend to be optimistic. Aiming at solving this problem，the three-dimensional anisotropic reservoir is converted into an equivalent isotropic reservoir through coordinate transformation. Then the flow region in the reservoir can be divided into external region with plane radial flow，middle region with plane linear flow and internal region with vertical radial flow based on typical seepage characteristics of the horizontal wells. A new comprehensive formula for calculating the productivity of horizontal wells in a three-dimensional anisotropic reservoir was derived by using the hydropower similarity principle and the equivalent flow resistance method. Comparison with the conventional horizontal well productivity formula shows that the flow resistance of the plane linear flow and its influence on the productivity gradually increase with the increase of the length of the horizontal well；the plane anisotropy has significant influence on the productivity of the horizontal well，and its changing law is controlled by the angle between horizontal well and the main permeability direction of the reservoir；the productivity of the horizontal well increases with the increase of the vertical and horizontal permeability ratio，and increases with the increase of the angle between the horizontal well and the main permeability direction of the reservoir.The application shows that the new formula has higher prediction accuracy with an average deviation less than 3%，and it can be used to guide horizontal well productivity prediction and optimization of horizontal section length and azimuth.