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.　©　2024　Chinese　Society　of　Theoretical　and　Applied　Mechanics.　All　rights　reserved.