It is generally believed that a long ellipsoid drainage area could be formed during the horizontal wells production，but the actual drainage area is just one part of the ellipsoid for the thin reservoirs that are sealed at the top and bottom.According to the seepage law of thin reservoirs，the three-dimensional flow problem was divided into two parts，including the horizontal plane elliptic flow far from the wellbore zone and the vertical plane radial flow near wellbore. Based on the principle of hydroelectricity similarity，Lame coefficient of the elliptical coordinates was introduced，and the productivity formula of horizontal wells was established for the thin reservoirs. But a complete ellipsoid drainage space could be formed in the thick reservoirs，which could be divided into a pseudo cylinder and two hemispheres. The productivity formula of horizontal wells in the thick reservoirs was established by introducing average short half axis theory and mass conservation principle as well as the equivalent flowing resistance method. Through the comparison and analysis of the deduced formula and the other five classical productivity formulas，the average deviation of the horizontal well productivity calculated by the new formula in the reservoirs of different thicknesses is less than 2%，which shows that the calculation results are reliable.