Abstract: The study and application of variable speed pig are very well developed in international natural gas pipeline inspection. However, the study on the speed regulation is rare in inland. With regard to the difficulties and features of variable speed technique, the paper summarizes the dynamics problems of pig, the current study status of bypass variable speed apparatus at home and abroad. Also, the structure, principle and the classification of bypass valves are analyzed, structure, performance and principle of bypass direct-acting block valve, rotating turn valve and butterfly valve are described. And meanwhile, the advantages and basic principles of brake mechanism of variable speed pig are introduced. The technical difficulty of variable speed pig is discussed. The paper points out that it is highly desired to develop the mechanical and control system design study, the theoretical and experimental study of control logic optimization.