Abstract: Short horizontal wells are those which lengths are much shorter than the size of drainage area. The conventional production rate equations are all established for short horizontal wells, which can not be used to calculate the production rates of long horizontal wells. So called long horizontal wells are those which lengths are almost the same with the size of drainage area. Based on the equivalent flow resistance theory, a production rate equation was derived in this paper, which can be used for production rate calculation of long horizontal wells, but can not be used for that of short horizontal wells, otherwise, the production rate may be overestimated. The production rate calculated by the long horizontal well equation is the lower limit of production rate calculated by horizontal well equations