Based on the principle of the mass conservation law and the momentum conservation principle，we established a mathematic model of drawdown along the horizontal wellbore and coupled the inˉflow in the wellbore with seepage flow in the reservoirs by using specific productivity index. Reservoir filtration calculation in the model is expressed by the steadyˉstate flow model of JOSHI，and calculation of frictional drawdown in wellbore takes into account the impact of the radial inflow. We also wrote program for the mathematic model，so as to calculate and analysis the effect of relevant parameters of the well upon its deliverability. The results show that acceleration drawdown is negligible and frictional pressureˉdrop is insignificant in bigger borehole with lower deliverability，while that gravitational drawdown caused by inclination of wellbores has a bigger impact upon the deliverability. The article provides a simple，easy and effective way for predicting the deliverability of steeped horizontal wells.