Matching fuel injection and airflow motion is critical for the optimization of fuel-air mixing and combustion process in diesel engines. In this study, the effects of swirl flow on liquid droplet motion and the selection of swirl ratio, which are known as the major concern in organizing airflow motion, were investigated based on theoretical analysis of droplet trajectories. The evaporating droplets with various initial conditions are assumed to be transported in a solid-body-like swirl field, and their trajectories were derived based on force analysis. To evaluate fuel-air mixing quality, a new parameter with respect to fuel vapor distribution was proposed. Based on this methodology, the effects of swirl velocity, droplet size, as well as liquid-gas density ratio on droplet trajectory were discussed under diesel-engine-like boundary conditions. It is found that, higher swirl velocity can stretch the droplet trajectory, while the influence of liquid-gas density ratio is similar but much less profound and deviates away from the cylinder center, especially for the lower swirl velocity. A moderately high swirl can improve fuel-air mixing through spreading the fuel widely in the combustion chamber, while an extremely high swirl might result in fuel vapor distribution overlap to form fuel rich region, which indicates that the critical value could be optimal. For 25 ~ 75 μm fuel droplets, the ideal swirl velocity was recommended to be 900 to 1300 rad ·s-1 when liquid-gas density ratio was taken as 840/20, which should also increase with liquid-gas density ratio.