To investigate the effects of specimen width and ignition position on the flame spread behavior of XPS insulation boards, a corresponding experimental apparatus was constructed, and its reliability was verified through repeatability tests. For B₂-grade XPS specimens of different widths, full-width linear ignition was applied at both the upper and lower sections, and relevant combustion and flame spread parameters were obtained. The results indicate that under concurrent flame spread conditions, the full-time maximum flame temperature fluctuation amplitude increases with specimen width; whereas under countercurrent flame spread conditions, this temperature remains stable. In concurrent flames, the instantaneous flame height exhibits a trend of first decreasing and then increasing with width, showing a weak correlation with time. In countercurrent flames, the instantaneous flame height tends to fluctuate and increase over time before stabilizing, and at the same width, the average flame height under countercurrent conditions is significantly higher than that under concurrent conditions. Regarding the flame spread rate, under concurrent flame spread, the linearly fitted spread rate shows no significant correlation with width variation for specimens of different widths. Under countercurrent flame spread, the linearly fitted spread rate is positively correlated with specimen width, and both the fluctuation range and mean value of the instantaneous spread rate under countercurrent conditions are smaller than those under concurrent conditions. At the same width, the average spread rate under concurrent flame spread is approximately 3.6 ± 0.3 times that under countercurrent flame spread. In terms of combustion behavior, molten material dripping occurs frequently in concurrent flames; whereas in countercurrent flames, molten material adheres to the flame and triggers sudden vertical flame propagation. Both phenomena may lead to severe fire hazards.