Abstract: The reversed mud acid is the mud acid whose concentration and volume of hydrofluoric acid is higher than that of hydrochloric acid, its treatment mechanism is identical to the mud acid. A kind of dissolution agent that can prevent fluoride from depositing or flocculating into cement was added into the acidic fluid of reversed mud acid in Dagang oilfield to avoid new plugging after acidizing the formation. The capacity which acidic fluid dissolute shale plugging was increased greatly when the formation plugging mainly made up of shale is removed by using hydrofluoric acid with high concentration and content, the shaly sand formation which mainly consist of clay cementation is improved or there is less carbon ate ingredient in mud plugging. So the reversed mud acid can improve formation permeability better than the conventional mud acid.