Abstract: In this paper, the basic structures of ball valves and the structures and adaptabilities of valve seats are firstly introduced, and then force analysis and formula derivation are conducted for double-piston and single-piston effect valve seats, so as to illustrate the sealing principles of ball valves. Moreover, the grease injection system which is often neglected in management is described in detail, its significance to the maintenance and sealing capacity of valves is analyzed, and an effective method is provided to diagnose if seal failure occurs. After the structures and sealing principles of ball valves are deeply analyzed, the possible reasons for ball valve seal failure are thoroughly investigated in the aspects of manufacture, field construction and operation management, based on practical applications. Finally, feasible means are proposed to check and treat the seal failures, and corresponding management suggestions are put forward.