To understand the mechanism of the transition from smoldering to flaming combustion of a flexible polyurethane foam, experiments were conducted with horizontally oriented foam layers under natural convection; this is a common scenario for a real fire. Temperature measurement, gas sampling, and thermal analysis were used to explore the transition's mechanism. It is seen that the exothermicity of the oxidation of the residual char left by the passage of the smolder wave front is much larger than that of the oxidation of the original foam material. The transition is determined mainly by the exothermic oxidation of the residual char and the availability of oxygen inside the foam. Furthermore, from the aspect of fire safety during the use of flexible polyurethane foam in upholstered furniture, a series of experiments was done on how the foam length, ignition power, moisture content in the foam and flame retardant influence the transition in the flexible polyurethane foam. The results show that these factors can affect the occurrence of transition from smoldering to flaming and also the transition time because of the oxidation of the residual char left by the smoldering.