Abstract: In terms of the time-invariant seepage of multi-lateral horizontal wells in oil reservoirs, a coupled model of the time-invariant seepage in reservoirs and the wellbore flow in multi-lateral horizontal wells was deduced with the full 3D analytical method based on the filtration mechanic theory. The pressure drop in barefoot completion multi-lateral horizontal wellbores was calculated using the improved Su method and a new approach, and distributions of pressure and productive indices in wellbores were analyzed and compared. At the same time, a computing method with the consideration of economic factors was proposed to optimize the length of the horizontal section for multi-lateral horizontal wells. The results derived from calculations and analyses of some real cases indicated that the flowing status within an individual branch of multi-lateral horizontal wells was similar to that in horizontal wells, the length of lateral branches of multi-lateral horizontal wells is not the longer, the better, but instead, it has an optimal value due to the friction effect. These conclusions are available for designing multi-lateral horizontal wells and forecasting the productivity of this kind of wells.