Abstract: The techniques of the high-speed schlieren photography, etc. were used to investigate the characteristics of the premixed methane-air flames with different equivalence ratios propagating through a closed tube. Based on the experimental results, the combustion behaviors and characteristics of the premixed flames were analyzed. Analyzed results show as follows: (1) the transformation process of the flame front bending to the unburned zone turning into bending to the burned zone during the flame propagation happens while the equivalence ratio volume approaches to 1, thus the typical tulip flame structure forms during the transformation; (2) the transformation process of the flame front does not happen while the equivalence ratio volume deviates from 1 to a certain extent, and the premixed flame shows the typical laminar combustion. (3) the tulip-flame structure forms in the period of the flame propagation velocity decreasing greatly and it can form only while the maximum deceleration is larger than a certain quantity; (4) the tulip-flame structure is a middle stage during the transformation of the premixed flame from the laminar combustion to the turbulent combustion.