The effects of heat release rate and opening size on the structure of spill flame beneath a horizontal projection are investigated. The lengths of the spill flame are obtained from digital videos combined with an image processing method. The theoretical model that describes the flow and combustion dynamics of spill flame under a horizontal projection is established. The air entrainment coefficient and the excess air ratio are determined by combing the theoretical model and the measured temperatures，which are about 0.12 and 3.7，respectively. The prediction formula for the flame length under a horizontal projection is thus obtained，which provides references for the passive fire prevention design by using horizontal projections.