Aiming at a thick carbonate reservoir（BU）in the Middle East Missan Oilfield，based on similarity theory，the artificial carbonate sample is used to simulate target reservoir，an large-scale physical model with size of 80 cm×80 cm×10.7 cm that is similar to the typical unit of target reservoir is built to study the water flooding performance，water flooding modes，and the development effect of different well patterns. Results show that the water flooding performance of thick carbonate reservoirs are mainly affected by the longitudinal heterogeneity and gravity，and are characterized by double peaksor single peaks. The water flooding mode is summarized as follows：the injected water moves longitudinally to the high-permeability layer near the water injection well and flow forward along the high-permeability layer. Away from the injection well，the top injected water moves downward under the effect of gravity，then moves along the lower high-permeability layer and breakthroughs to the bottom of the production well to form a secondary bottom water. Finally，the secondary bottom water moves upward，causing the production well to be completely watered. Based on above mode，the remaining oil mainly distributes in the upper layers far from the water injection well；correspondingly，the horizontal well infill well pattern shows a higher control over the reserves of the upper layer than other well patterns，hence it can lead to the higher recovery and development efficiency.