Abstract: The process of flame acceleration and deflagration to detonation (DDT) of ethylene/air premixed gas in three widths (20 mm, 10 mm and 5 mm) channels were studied by numerical simulation. It is found that the smaller the channel width d is, the earlier DDT occurs, but the length-diameter ratio of DDT increases with the decrease of d. In the narrow channel, the DDT is easily caused by the formation of hot spots in the boundary layer. In the wide channel, it is more likely to cause local explosion inside the pipe to form DDT. When d =20 mm, the detonation wave propagates in the mode of multi-head. When d =10 mm, the detonation wave has a single head rotating structure. When d =5 mm, no cellular structure appears, and only large pressure pulse-like cycles can be observed. The detonation velocity propagates at the speed of 1 166 m/s to 1 822 m/s, and the average velocity is 1 380 m/s.