Abstract: The gasi N1-N21 reservoir is the biggest production res-ervoir of Qinghai oilfield,the main problem of the reservoir is that nature depression decline quickly,water displacement effect is lower,water flooding is unclear and recognition is low.We carried on detailed water flood analysis with factional flow curve method and gained water flood front,front water contend saturation,average water saturation of different stage.On the base of that,we can calculate the flooding efficiency,which provided valuable data to improve the control ability of development and final recovery rate.Now the reservoir is going through second development and adjusting.Factional flow curve method is very useful to oil field development and proved good effect.