Plateau diesel engines suffer from increased fuel consumption and deteriorating emissions. These problems were mainly caused by the combustion worsening inside the chamber. However, limited research has been done specifically on the effect of altitude on the combustion process of diesel engine cycles, which would obviate the feasibility of optimizing high altitude engines. As a result, the goal of this study was to apply the zero-dimensional modeling approach to deeply analyze the influence of altitude on diesel engine combustion. A triple Wiebe function model was calibrated based on the experimental results of a turbocharged direct injection compression ignition engine operating at simulated atmospheric conditions from sea level to 5000 meters, where only the effect of pressure reduction was considered, but the intake temperature and oxygen to nitrogen ratio were kept constant. The analyses indicated that the increase in altitude lengthened the ignition delay, resulting in more fuel fraction being burned in the premixed combustion stage and therefore extending the duration of this phase. As for the main mixing-controlled combustion phase, operation at high altitude retarded the combustion initiation angle, advanced the combustion end angle, shortened the burning duration, and reduced the diesel mass burned in this stage. Moreover, the higher altitude operation increased the energy release and prolonged the duration of the late combustion period, which was detrimental to clean emissions. All these impacts contributed to the reduced thermal and combustion efficiency of the highland engines. However, the engine phasing did not change with increasing altitude, suggesting that it was mainly the combustion degradation that caused the reduction in power output. Consequently, finding solutions to improve the spray formation quality or the spray spatial distribution in lower density backgrounds is the key to compensate for the altitude negative effects.