To effectively block the vertical spread of flames in concave high-rise buildings, this study explores the fusion height of window flames and the temperature distribution law by changing the window dimensions of concave high-rise buildings, and then reasonably sets the fire prevention and isolation zone based on the flame fusion height. Numerical simulation analysis is conducted on a certain actual concave high-rise building using the fire simulation software PyroSim. Considering the condition of simultaneous combustion of three vertical windows, the window dimensions (height × width) selected are 1.5 m × 1.2 m, 1.5 m × 1.8 m, and 1.8 m × 2.1 m respectively, and the isotherms of temperature distribution and temperature curves of the model under different conditions are analyzed. According to the numerical simulation, when the dangerous temperature of 540 ℃ is reached, compared with the window dimension of 1.5 m × 1.2 m, the upward fusion height of the flames for the window dimensions of 1.5 m × 1.8 m and 1.8 m × 2.1 m increases by 2.1 m and 3.5 m respectively, and the downward fusion height of the flames increases by 0.35 m and 0.35 m respectively. The results show that for concave high-rise buildings, with the increase of the ventilation factor, the fusion height of the flames becomes larger and larger; and the degree of downward flame spread is relatively small and can be ignored. Considering comprehensively, in order to effectively suppress the mutual fusion of flames between the floors where the fire occurs, the height of the fire prevention and isolation zone can be set to 9 m.