In this paper,first,the dimensionless expressions of the smoke backlayering length and the criticalvelocity in metro tunnels were deduced via a dimensional analysis.Then,nine tunnels with different cross sectionswere employed according to the survey results from the tunnel form of the Metro Line 6 in Chongqing,China,andthe smoke flow in the tunnels was analyzed with FDS5.5.Finally,the prediction models of the smoke backlayeringlength and the critical velocity were established,and a 1/10 scale model tunnel was built to carry on small- scalemodel experiments.The results show that the results obtained by the proposed prediction models accord well withthose of small- scale model experiments,and that the critical velocity predicted by the existing method is underesti-mated.