The S-shape inlet is generally the main reason for the occurrence of swirl inlet distortion that is seriously concerned by the manufactures of fighters. The non-uniform inlet combined with large-scale bulk swirl, small-scale tight-wound swirl, or both of them may greatly impact the performance of aero-engines. Focusing on the swirl patterns and the effects of swirl flow on the compression system, many attempts have been carried out. However, there is still no general and quantitative criterion for assessing the swirl pattern and intensity to serve the further performance or stability analysis. In this paper, a vorticity-based method was used to quantify the swirl inlet distortion caused by different S-inlet ducts. Three series of S-ducts with different offset distances and different lengths were designed. Numerical investigations on the intensity and form of swirl distortion were carried out to achieve quantitative results.