Knowledge of vented flame propagation is of significance in the design of protective work against the effects of a gas explosion. Since there is little attention paid to the damage caused by the vented flame, the propagation characteristics of a vented flame in an external space were studied. Vented explosion experiments were conducted in a custom-designed chamber under three concentrations of ethylene/air mixtures and numerical simulation was also used. The propagation of a vented flame in an external space mainly depends on the combustion with unburnt gas and four typical flame profiles due to the effect of Darrieus-Landau flame instability and diffusional-thermal instability are observed. At different stages of propagation, characteristics such as velocity, stretch rate, and distance are all different. The reach of the vented flame can be expressed as an exponential relationship with time and the acceleration exponent is 0.65, which indicates that the vented flame is essentially undergoing spherical decelerating propagation under unconfined deflagration conditions. The numerical simulation applied is in good agreement with the experiments, which provides a basis for the analysis of the external flow field. (C) 2019 Elsevier Ltd. All rights reserved.