The reduced atmospheric pressure and oxygen mass concentration at high altitudes have brought tremendous problems to the operation of gas‐fired boiler, mainly due to their influence on gas combustion and flow within furnace. Boilers designed for plateau region generally experience insufficient output, incomplete combustion, and reduced thermal efficiency when operating at high altitudes. However, the impacts from increasing altitude to the combustion characteristics within furnace are still ambiguous, which greatly hampers the boiler design for high‐altitude area. In this paper, the influences of altitude on the gas combustion characteristics within furnace were explored by using the computational fluid dynamics (CFD) method. The simulation shows that when altitude rises from 0 to 4000 m, the flame length and width increase as well as the temperature within furnace, but the burnout rate decreases due to the reduced residence time. The furnace length and diameter of the boiler designed for high altitudes should be appropriately increased to avoid the high temperature flame from scouring the back tube sheet and the furnace liner. While the CO emissions are generally very low, an increasing trend with altitude was observed. In addition, for each 1000 m of altitude rise, the NOx emissions increase by about 14 mg·m−3 at 100% loads and α = 1.05. The present work reveals the influence of altitude on combustion characteristics within gas‐fired boiler, which can guide the design and operation of the gas‐fired boiler at high altitudes.