Abstract: As their radii are close to each other, the mixing mechanism of dual-species gas in pipe is analyzed and the effect of the purging velocity on length of mixed region is numerically studied. The result indicates that the mixture length increases quickly in the initial purging state and then changes slowly with the increasing of the length of purged pipe cleaning pig and the radius of the circle pipe is small. Lubrication theory of low Reynolds Number is used to form a hydraulic pipe cleaning flow mathematical model.