In order to solve the problems existing in the design and processing of helical surfaces commonly used in the structure of gun automat, its application scheme, theoretical characteristics and processing calculation method are analyzed. This paper analyses 3 kinds of common helical surface application forms, studies the characteristics of helical surface, deduces and establishes theoretical formulas for helical surface processing and clearance calculation, some of which have achieved good results in engineering practice. The results show that the design is practically and feasibly, which can provide reference for the design and processing of helical surface of gun automat.