[2] investigated the influence of altitude on a natural gas stove's CO emission in Chongqing and Lhasa, and found that oxygen-lacking conditions in Lhasa promoted the formation of CO. But [PERSON] [15] observed the opposite conclusion about CO. Smoke emission is a signature of incomplete combustion, and the minimum fuel flow rate (or flame height) is defined as the smoke point at which the flame just begins to emit smoke. There are limited experimental studies published regarding the smoke points of gaseous fuels, and even fewer publications have focused on the effects of sub-atmospheric pressure. [PERSON] et al. [16] tested the smoke point of methane over a range of pressures from 101 to 1616 kPa, and found that the flame height at the smoke point decreased with increasing pressure. [PERSON] et al. [17] found that smoke point flame lengths and fuel flow rates can be increased by increasing air/fuel-stream velocity ratios, and the effect was most pronounced at low pressures. [PERSON] et al. [18] investigated ethylene and propane laminar jet diffusion flames burning in still and slightly vitiated air at pressures of 35-130 kPa. The smoke point flame height is roughly inversely proportional to pressure. [PERSON] et al. [19] investigated the smoke point of acetylene laminar jet diffusion flames under sub-atmospheric pressures. The measured smoke point flame height, fuel flow rate, and residence time were found to increase with decreasing pressure. However, due to the low carbon content of methane, little literature considered how sub-atmospheric pressure affects the methane smoke point. To sum up, although the above studies have investigated the influences of low air pressure on gas combustion in an open environment, the relevant work for methane combustion behavior within a confined space under sub-atmospheric pressure is yet to be reported in the literature. Compared to methane combustion in an open environment, O\({}_{2}\) is accurately controlled, and the heat loss is smaller within a confined space. In addition, the radiation heat transfer between flame and wall within a confined space is the main heat-transfer method, and the flame heat transfer has a significant impact on the flow and chemical reaction process. The reduced flame radiation capacity under low pressure will inevitably lead to different combustion characteristics than those in an open environment. Hence, the guidance previous studies can provide is limited, and it is significant to further study how sub-atmospheric pressure affects gas combustion characteristics within a confined space. To study the influence of sub-atmospheric pressure on combustion behavior of combustible substances, it is necessary to obtain the low-atmospheric pressure required for experiments. Some scholars usually adopt the following two methods: (1) Establish the same experimental platform at high- and low-altitude areas [4; 10; 18]. (2) Simulate a low-atmospheric pressure at low-altitude areas [1; 12; 19; 20]. The latter is widely used because of its convenience in conducting repetitive experiments, lower experimental costs, and the ability to achieve a wider pressure range. In addition, the similar experimental results were obtained by using the above two methods. Therefore, by using the second method, we designed and built a simulated low-air-pressure experimental platform in Xi'an. With the above considerations, the objective of the current work is to optically study the effects of sub-atmospheric pressure on combustion characteristics of a methane diffusion flame within a furnace, especially the similarities and differences between fuel rich and lean conditions. Specifically, the effects of pressure (ranging from 61.66 to 97.75 kPa) and excess air coefficients (\(\alpha\), ranging from 0.8 to 1.3) on temperature distribution, flame appearance, smoke point, CO emission, and NO\({}_{x}\) emission are compared.