Fire behavior of flexible polyurethane foam (FPUF) at different sample thicknesses and ignition positioning was investigated. Effects on flame height, mass loss rate and other parameters were tested, and the flame propagation mechanism was analyzed. A method for predicting equivalent combustion diameter (D) values of the dynamic change of liquid pool at different positions is proposed. Combined with data of sample mass loss rate, flame height can be predicted. Based on a transition state model, a method for predicting the fire risk of FPUF in late stage combustion by calculating the generation time of polyols is proposed. With edge ignition, FPUF burning produces an inclined surface during the combustion process which enhances the length of the preheating zone by means of heat conduction and heat radiation. Flame spread rate (FSV) in FPUF with edge ignition was greater than with center point ignition.