Abstract: This paper presents an improved algorithm which combines the advantages of Chorin's vortex method, Vortex-in-Cell method and rapid elliptic algorithm. In the present method, the numbers and positions of the discrete vortices are fixed on meshes, and the vor-ticities of the discrete vortices are adjusted with the time advances. Numerical simulations of flow structure around a circular cylinder in oscillatory flow, ranging in Kc from 1 to 5 and β from 10 to 1665, are presented in the paper. The comparison of ...