Abstract: The mixing mechanism of dual-species gas in pipe is analyzed, and the effect of the purging velocity on the length of mixed region is numerically studied. The result indicates that the mixture length increases very fast in initial purging state and then changes slowly with the increasing of length of the pipe purged. The concentration in the mixed region appears asymmetric in distribution profile along axial direction, which has a characteristic of short head and long tail. Purging velocity is one of the key parameters during purging process. The length of mixed region is shorter during turbulent flow than that of laminar flow. Velocity increase has less effect on reducing the length of mixed region.