Flame spread over the surface of the thick PMMA sheet with heat-source backing in an opposed air flow is investigated experimentally. The flow velocity (U=50–150 cm/s), fuel thickness (δ=8.2–17.4 mm), and fuel backing temperature (Tb=60–150 °C) are varied to examine their effects on the flame spread rate over the thick PMMA. The results show that the rate of flame spread over the fuel with heat-source backing is significantly higher than that without heat-source backing. The local flame spread rate decreases along thedistance of flame propagation due to the decrease in the solid fuel temperature and the temperature of incoming oxidizer ahead of the flame front. In addition, with the same fuel backing temperature, the flame spread rate decreases with increasing the local Biot number of the solid fuel. Moreover, lowering the convective heat transfer of the thermal-backing fuel could approach the flame spread behavior to be that of faster chemical reaction rate. Apower-law correlation for the fiame spread rate over the thick PMMA with heat-source backing is further developed in terms of the fuel backing temperature, and the fuel Biot number, which may be helpful for controlling the flame spread in accidental fires and a useful reference for theoretic aspects.