Micro-mixing combustion is a kind of potential technology to reduce pollutant emissions, and the microtube structure as a basic unit has significant influence on combustion characteristics. Taking the single microtube model burner as the object, the computational fluid dynamics (CFD) method is employed to analyze methane micro-mixing combustion characteristics with different microtube diameters, rear-section lengths, and chamber-to-tube area ratios, under the air-preheated and pressurized condition and the atmospheric condition. The numerical simulation results show that, the flame length increases obviously under the atmospheric condition, and the increase rate is greatly affected by the microtube diameter and the area ratio. With the increase of microtube diameter, the flow and temperature fields in the flame chamber are similar, and the dimensionless flame length does not change significantly. However, when the microtube diameter is smaller, the change of flame length is smaller between two working conditions, and the flexibility is better. With the increase of microtube rear-section length, the turbulence intensity at the microtube outlet reduces greatly, and the flame length increases obviously, but the change trend of flame length is consistent under two working conditions. With the increase of chamber-to-tube area ratio, the flame length decreases at first and then increases. When the area ratio is 4.0~9.0, the flame length is relatively short, and the combustion performance is relatively good. The results have reference value for the gas turbine combustor, and also have certain reference significance for free jet flame.