Abstract: Southern reservoir of Dagang oilfield has characteristics of low permeability,low porosity,strong sensitivity,and high salinity formation water,which causes water injection pressure go up at an average speed of 0.8~1.5 MPa,and brings trouble to the production safety.By analyzing the influencing factors such as reservoir lithology,physical properties,and water quality etc,and combined with the characteristics of the water injection well curve for the different kinds of reservoir water injection well,the acidification and acid fracturing technology of depressurization have been optimized,which achieves obvious effect.