In this paper, the atomization process and spray characteristics of pressure-swirl nozzle in Stirling engine were investigated by the high-speed video imaging technique, and the effect of gas ejecting and injection pressure on the atomozation was analyzed. Experimental results showed that the atomization was poor and duration was short in the initial spray stage, while the main spray stage was characterized by air vortex; the spray angle increased quickly with the injection time and then decreased and finally stablized; the tip penetration increased with the injection time before reaching steady state, and the spray cone angle expanded with the increase of injection pressure. With the increase of ejecting air flow, the spray diffusion range was enlarged and the droplet distribution was improved, making the diesel and air better mixed.