This work investigated the effects of an in-cylinder active thermo-atmosphere environment (ATAE) on the diesel engine combustion and emissions in a single-cylinder diesel engine. Port-fuel injection of n-heptane was used to prepare the lean fuel/air mixture, and active radicals and heat release were found during the low-temperature reaction (LTR) and high-temperature reaction (HTR). The premixed ratio of n-heptane was used to denote the ATAE intensity. The effects of ATAE intensity and fuel delivery angle of directly injected diesel fuel on the combustion and emissions were evaluated. The experimental results reveal that, as the premixed ratio of n-heptane increases, both the maximum values of the heat release rate (HRR) in the LTR and HTR increase, which means that the ATAE intensity increases. With the increase of the ATAE intensity, the peak value of the diffusion combustion decreases and NO, emissions and smoke opacity reduce substantially at first and attain to the lowest levels at a certain point. Once the ATAE intensity exceeds this critical value, NO, emissions begin to increase monotonously but the smoke opacity increases to a peak point and then begins to further decrease. Overall, under those operating conditions, the optimized ATAE intensity is about 20-30%.