Abstract: Due to its capability to manipulate small volume fluids in microchannels, microfluidics has emerged as a novel platform for the manipulation of particles and cells. The method of using inertial migration and vortex trapping of particles in microcavities has become an important microfluidic particle manipulation technique. Currently, the number of captured particles accommodated by the microcavity is not high, which restricts the sorting efficiency of this method. To improve the holding capacity of microcavities, different round microcavities were designed, and their holding characteristics for sorted particles were investigated by using high-speed microimaging technology and numerical simulation. The results show that under the same inlet Reynolds number (Re = 37 ~ 555), the number of captured particles in a round microcavity with a bottom chamber can be enhanced by 45% compared with that in an ordinary round microcavity. The reason is that the addition of the bottom chamber causes the vortex streamlines to extend downward, creating a deeper "U-shaped" structure to hold more particles. At Re = 482, the holding capacity in a round cavity with a diameter of 500 μm increases by 93.9% compared to that with a diameter of 600 μm. The reason is that the particle motion track in the round microcavity with a diameter of 500 μm is more consistent with the streamlines, and the ratio of particle trajectory area to the cavity area is up to 97%. The concentration of particles collected from the side channel shows an overall trend of slowly increasing and then decreasing with increasing Re. The maximum concentration of 20 μm particles is 126.7 times higher than that in the initial suspension. The orbital motion of particles in different cavities is influenced by the vortex flow field characteristics, particle properties, particle interactions and wall confined effect. The research results could provide useful guidance for the design of microcavity structures and the improvement of particle sorting performance.