Abstract: Considering equivalent flow resistances and non?piston water flooding, a multilateral horizontal well can be equivalent to a vertical well with enlarged borehole, and the flow region is divided into three resistance zones. The productivity formula, water breakthrough time and sweep efficiency for well pattern with vertical injector and multilateral horizontal producer are derived. Using the model described in this paper, the effects of main wellbore length, branch length, branch angle and well spacing on the water breakthrough time and sweep efficiency are analyzed. The results indicate that the water breakthrough time decreases with permeability increasing. Under the same permeability condition, the water breakthrough time and sweep efficiency decrease with the main wellbore length increasing, and increase with well spacing increasing; Branch length and branch angle have negligible effect on water breakthrough time and sweep efficiency. In conclusion, five?spot well pattern is optimized after full consideration of water breakthrough time and sweep efficiency