The swirl ratio significantly affects engine output power, fuel economy, and exhaust emissions in diesel engines. Thus, the selection of swirl ratio is desired in diesel engines. In this paper, the selection of swirl ratio in diesel engine was studied based on the theoretical simplified analysis of droplet trajectory. Firstly, the equation of droplet trajectory was derived through the analyze of the forced state of single droplet in a solid body like swirl field and an evaluation parameter for fuel vapor distribution was proposed. Then the effects of swirl rotating speed, the diameter of fuel droplet and the density ratio of fuel droplet and ambient gas on droplet trajectory and fuel vapor distribution were discussed. On the basis of those, the matching relation of these three variables was investigated under the boundary conditions of diesel engine. The results show that, the higher swirl rotating speed can stretch the droplet trajectory observably and deviate it from cylinder center, while the influence of density ratio of fuel droplet and ambient gas is opposite and significantly lower than that of swirl rotating speed. The higher swirl rotating speed can improve the mixture of fuel vapor with ambient gas, but the excessive swirl rotating speed could lead to the overlap of fuel vapor distribution. For 20 –100μm fuel droplets, the ideal swirl rotating speed was recommended from 800 to 1200 rad/s. And the density ratio of droplet and gas is smaller, the ideal swirl rotating speed is higher.