Based on the fishbone diagram analysis for gun drill wearing, the major technological factors influencing gun drill serve life in the machining process of the main oil gallery hole of a engine cylinder head were introduced. The serious problems in gun drill deep hole drilling were chip breaking and chip removal. In view of the existing problems, the shape and geometric parameters of the cutting edge and cutting parameters were analyzed and improved. The application results show that the chip can be break well and discharged, also the gun drill life and the production efficiency can be increased greatly.