Based on seepage characteristics, the seepage flow status of horizontal wells in formation is simulated by applying physical methods. This paper proposes a new drainage area in a horizontal well. A new productivity equation for horizontal wells is derived with the equivalent seepage flow resistance principle. The new productivity equation can determine the influence of the distance from the top level of the horizontal well for bottom water reservoir on well productivity and is applied to validate the new horizontal wells in Bohai oilfield Q. With the actual parameters of the oilfield, the production differential pressure calculated by the new formula is close to the field test results.The results are in high conformity, withaverage error of less than 10%. Therefore, the new formula is reliable and of strong applicability. The horizontal well production capacity based on the new formula is relatively reasonable, which provides theoretical supports for the prediction of oilfield horizontal well productivity.