The leading edge slat takes an important role to increase aircraft lift and to improve flight performance during taking off and landing . The new type slat actuating unit--pinion-rackis used greatly due to ingenious design ,little occupied room and light weight. In design of pinion and big arc rack ,definition of parameters,design of key details ,coordination for full-scale actuating units ,calculation and analysis of fault-loads make of important and difficult points of units design. The pinion-rack actuating units which is adopted the method hereinbefore to design and research have been applied on aircrafts successfully.