With increasingly stringent emission regulations, the cold start emissions have become more important than ever. Using a low compression ratio is a feasible way to improve a heavy-duty engine’s efficiency and emissions. However, cold start performance restricts the development of this technology, especially at high altitudes. In response, we conducted a study of the emissions of a heavy-duty low-compression-ratio diesel engine during start-up process at different altitudes. A plateau simulation system controlled the inlet and exhaust pressure to create altitude environments of 0 m, 1000 m, 2000 m, 3000 m, 3750 m and 4500 m. The gas, particulate and volatile organic compound (VOC) emissions were analyzed with speed and cycle during the start-up process. The results indicated that cold start performance and combustion characteristics became worse as altitudes increased. The gas and particulate emissions of carbon monoxide (CO), carbon dioxide (CO2), total hydrocarbon (THC) and nitrous oxide (NOX) almost all increased as the engine speed and altitude increased, and was much higher than in idle conditions. The PN and PM emissions in each particle diameter also increased as the altitude increased, which was the same as the nucleation mode and the accumulation mode particles. VOC emissions were also measured, which increased during the start-up process as altitudes increased.