Upward flame spread characteristics over extruded polystyrene (XPS) foams with different porosities has been analyzed through experiments. In this paper, the average flame height and average maximum flame temperature first rise and then drop with increasing porosity, which is affected by the fuel and oxygen competition mechanisms. For P (the porosity of XPS samples) ≤ 35%, the positive effect of pores plays a dominant role; the average flame height and average maximum flame temperature increase with the increasing porosity. While the negative effect of pores plays a dominant role when P > 35%, causing the average flame height and average maximum flame temperature decrease with the increasing porosity. Modeling and experiments were conducted to study the heat flux from flame. The value of radiation is obviously higher than convection through formula derivation and the experimental results have high similarity with the theoretical results.