Abstract: The fire propagation characteristics of polyurethane foam (PU) under the vertical external angle configuration was studied experimentally by a self-built experimental platform. The results show that the down-flow fire propagation behavior of polyurethane foam is mainly affected by the combustion characteristics, heat transfer mechanism, confined configuration of facade and corner structure. In the process of fire spreading, the early combustion reaction is slow, which leads to the thickening of the carbon layer and the formation of a transient quenching zone bounded by the melting interval. Under the action of the angular structure, the inclination angle of the pyrolysis front first decreases, then increases and then decreases. The typical characteristic parameters of fire spread, including average fire spread rate, combustion rate and average flame height, show periodic oscillation law under the joint action of droplet effect and air enrolling. For corner structure in terms of area, fire spread heat transfer mechanism, heat transfer and chemical equivalent ratio is mainly decided by the structure factor, structure factor in research analysis. On the basis of the intrinsic mechanism of the change, combined with air entrainment cause flame thrust and heat dissipation mechanism, analyzes three kinds of characteristic parameters. The average flame height distributed in three segment pattern during the flame initiation period, and follows the formula of hf（t）=-95.43+1.1.93exp（0.025t）. The average fire spread speed and combustion rate both exhibit a sudden change from increasing to decreasing during the corner stage.

Key words: downstream fire spread, corner area, polyurethane foam, heat-transfer mechanism, elevation of fire