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[Objectives]The confined spaces such as the ship's engine room usually has a horizontal opening at the ceiling,and the particularity of the position and shape of the opening results in a significant difference between the combustion characteristics of the fire in such space and those of the building fire. In order to understand the fire characteristics of the horizontal ceiling opening of the ship's engine room,[Methods]the combustion characteristics of the fire such as the flame-extinguishing characteristics, flame shape and combustion rate in the confined space fire with a horizontal ceiling opening were experimentally studied by changing the ceiling opening area of the simulated test chamber and the size of the heptane oil pool,[Results]According to the reason for fire extinction,the development process of confined space fire are divided into "oxygen-lack extinguishing" mode caused by insufficient O2 concentration and "fuel-lack extinguishing" mode caused by depletion of combustibles. In the "oxygen-lack" mode,the flue gas mixture is entrained into the flame to participate in the combustion process. The critical O2 concentration for self-extinguishing of flame is between 13% and 16.5%,and the shape of the flame changes from a stable combustion shape to a state of being out of the oil pool and keeping moving. In the "fuel-lack" extinguishing mode,the flame is relatively stable in shape,and the combustion rate in the late combustion period is significantly increased due to boiling combustion.[Conclusions]The results show that the combustion characteristics of the fire in the confined space with ceiling opening are of great significance for understanding the development process of the cabin fire and provide theoretical support for the cabin fire suppression.