Abstract: The BZ Oilfeld in Bohai area is a typical narrow channel sand oilfeld developed by water injection. At present, there is a very high wellhead pressure in part of the injection wells. It is urgently necessary to analyze the cause of high wellhead pressure and the injection capacity of the injection wells in the oilfeld. The authors, by adopting the theory of percolation mechanics and taking consideration of the variation of the thickness and permeability of the narrow channel reservoirs, derived the calculation formula of the injection pressure in the narrow channel reservoirs. The results are very close to the numerical simulation. The formula analysis demonstrated that the high wellhead pressure was caused by the development mode of thin layer injection and thick layer production in the narrow channel reservoirs. A theory plate of maximum water injection capacity was plotted by the formula of narrow channel reservoirs, which can be used to guide the proposing of acidizing measures and has achieved good application effect.