A novel ion-extraction method named side extraction, i.e., extracting ions from the direction perpendicular to that of plasma expansion, is proposed in this paper, which can be used to filter ions or macro-particles for vacuum arc ion sources. Particle-in-cell simulations were performed to study filtering effects of side extraction on ions with different mass, different charges, and different initial kinetic energies. The results indicate that side extraction can separate ions by their charge states or initial kinetic energies and the extracting efficiency for different ion species is identical. Influences of extraction voltages, extraction interval, and the aspect ratio of the extraction aperture on ion filtering effects were also studied, which suggests that lower extraction voltages and longer extraction interval are beneficial for ion filtering and a high-aspect-ratio aperture contributes to the increase in the ratio of lighter ions in extracted beams. A theoretical analysis on the deflected distance of beams in the y direction was made and shows good agreement with the simulation results.