Abstract: Research on the status of motion rule of the pig for gas transmission pipelines is described. It is pointed out that most motion models of the pigs at present adopt the steady state and unsteady state assumptions as application precondition, therefore, a greatdifference exists between the assumption and true gas pipeline conditions. F. Esmaeilzadeh proposes an unsteady state motion model of the pigs. The model uses MOC and Runge Kutta method to solve state equation, continuity equation, momentum equation and energy equation, and calculation results can perfectly forecast traveling speed of the pig in a pipeline, which is of instructivesignificance in pigging operation of gas transmission pipelines.