This paper explores the lateral spread of facade flame in conjoined high-rise buildings under different air humidity conditions. Numerical simulation is carried out on the combined high-rise building model with PyroSim. In the case of 37%, 63% and 86% air humidity, the temperature distribution isotherms and temperature curves under the simultaneous combustion of the model in different numbers of transverse continuous windows are analyzed. Based on the numerical simulation results, when the dangerous temperature reaches 540 ℃ under different air humidity conditions, the flame fusion width of three transverse continuous windows is 18.1 m−18.4 m higher than the two transverse continuous windows, and the flame fusion width of four transverse continuous windows is 1.7 m−3.5 m higher than the three transverse continuous windows. With the increase in the number of transverse continuous combustion windows, the air humidity has a greater effect at the flame fusion width. When the humidity is 37%−63%, the flame fusion width increases with the humidity; when the humidity is 63%−86%, the flame fusion width decreases; hence, the humidity of 63% has a greater impact on the flame fusion width than the humidity of 37% and 86%. The results show that the growth of flame fusion width of three transverse continuous windows is much greater than that of the two transverse continuous windows. It is suggested to set fire-resistant windows at every two windows to prevent the lateral spread of flame.