Abstract: To investigate the exhaust flow characteristics of diesel engine in high power density (HPD) condition, 1-D thermal dynamic model for a single-cylinder diesel engine was built. The control parameters and thermal dynamic parameters with the engine speed of 3000 r/min were simulated. The results of simulation were in accordance with the experiment data. The effect of increasing power density on exhaust flow was analyzed using validated model. The results showed that the corresponding crank angle of supercritical exhaust state were obviously improved with the increase of engine speed, peaked at 128oCA in 4200 r/min condition, and the exhaust indicate power was enhanced markedly. The area of exhaust flow became the main factor affecting exhaust performance of diesel engine with high power density (HPD). With the increase of exhaust throat diameter, the corresponding supercritical angle diminished and the absolute value of indicated exhaust power reduced. This research can provide theoretical reference for optimizing HPD diesel engine exhaust system.