The vertical upward and downward flame spread characteristics of insulation material EPS in the concave structure were studied by building a small-scale flame spread experimental platform. The changes of parameters such as flame structure characteristics, flame spread speed, mass loss rate, flame temperature were comparatively analyzed. The research results showed that in the process of vertical upward spreading, EPS appeared stagnant combustion; this was mainly caused by the large gas flow generated by the chimney effect. However, in the process of vertical downward spreading, the flame spread speed was obviously accelerated due to the increase of EPS accumulated in the process of flame spread and the effect of the reverse stack effect. During the upward spreading process, the length of the pyrolysis zone was significantly longer than the pyrolysis zone spreading downwards. At the same time, the flame was basically inclined to both sides of the concave groove, and the molten dripping of the material can be clearly seen, but it was spreading downwards almost no molten spilled liquid. The mass loss rate in the process of flame spread was affected by the flame height to a great extent, which was mainly manifested in two stages: initial oscillation stage and stable oscillation stage. The influence of the stack effect on it was transient and temporary. In the unburned area, the flame temperature experienced two temperature peaks, and the first peak was larger than the second peak in the case of upward spread; while the first peak was smaller than the second peak for downward propagation. This was mainly due to the structure of the flame and the directional characteristics of the chimney effect, resulting in the generation of two different peak sizes. The research results of this work can provide early theoretical and reference value for the study of flame spread characteristics under actual concave structures.