Abstract: Abstract: A high-altitude environment has posed a great challenge to the performance of conventional diesel engines. The reduction of intake thin air can lead to the deterioration of the combustion process, when the diesel engine is operating at a high altitude. Particularly, the power performance and thermal efficiency can be reduced significantly during this time, together with the much larger emissions of the diesel engine. Meanwhile, more stringent requirements have been released for the emission of diesel engines at a high altitude under the control conditions in China's Stage 6 Emission Standard of light-duty vehicles. Therefore, much more attention has been paid to methanol fuel, in order to improve the performance of diesel engines at a high altitude, particularly considering the most promising low-carbon clean fuel. This study aims to investigate the influence of methanol substitution percentage (MSP) on combustion characteristics in a diesel/methanol dual fuel (DMDF) engine at a high altitude. A systematic experiment was also performed with a high-pressure common rail diesel engine. Methanol was injected into the intake manifold through a methanol nozzle mounted on the intake manifold in the diesel engine. Three working conditions of A, B, and C were selected, where the working condition A: the speed of 1 200 r/min, 50% load; the working condition B: the speed of 1 800 r/min, 50% load; the working condition C: the speed of 2 200 r/min, 50% load. A high-altitude atmosphere testing system was designed for the experimental environment of three altitudes, including 10, 700, and 2 400 m. The in-cylinder pressure, fuel consumption, and emissions were also measured during the process. Some key combustion parameters were calculated according to the data of in-cylinder pressure, such as the heat release rate, pressure rise rate, and combustion duration. Correspondingly, the maximum MSP was obtained under the three working conditions at the three altitudes in the calibration test. The results showed that the maximum in-cylinder pressure increased 5.74%-26.14%, compared with diesel (D100), when the MSP of three working conditions reached the maximum at the three altitudes. The first peak value of heat release rate increased 116.98%-234.83% at various altitudes with the increment of MSP, where the crank angle was postponed by 1.5-5.0 °CA. At the same time, the premixed combustion proportion expanded significantly. Furthermore, the variation in the pressure rise rate was similar to that in the heat release rate. The maximum pressure rise rate increased by 49.99%-211.97%, while the curve of pressure rise rate gradually changed from double peak to single peak. The maximum in-cylinder temperature increased 3.99%-8.53% at the three altitudes, compared with D100, when the MSP reached the maximum. It infers that the increase of in-cylinder temperature was greater, as the altitude increased. A combination of parameters was achieved under the maximum MSP of three working conditions, where the ignition delay period was extended by 1.00-2.50 °CA, while the combustion duration was shortened by 9.80-15.30 °CA, and the combustion center was advanced by 2.10-7.90 °CA, indicating the rise of altitude further aggravated this tendency. In addition, the brake thermal efficiency (BTE) increased by 0.64%~1.82%. The coefficient of variation (CV) of peak in-cylinder pressure (COVPP), and indicated mean effective pressure (COVIMEP) also increased with the increment of MSP at different altitudes and working conditions. There was a downward trend of COVIMEP at the same MSP, with the increment of altitude at a high speed and high MSP (>30%). Both COVPP and COVIMEP could also be controlled between 0.6% and 3.5% during the process. The soot decreased by 26.94%-74.05%, while the NOX emission decreased by 4.23%-37.97% at different altitudes, compared with D100, when the MSP reached the maximum. An optimal MSP was achieved for the in-cylinder combustion of the DMDF engine, thereby improving the thermal efficiency and greatly decreasing the soot and NOX emission simultaneously at high altitudes. The DMDF engine can be expected to apply for a large methanol substitution percentage (≤50%), to improve the combustion process in the cylinder at different altitudes. The finding can contribute to the power performance in the high efficiency, further to clean the combustion of engines at high altitudes.