An　unreinforced　masonry　(URM)　structure　damaged　by　a　strong　earthquake　may　be　threatened　by　subsequent　aftershocks　in　an　unrepaired　state.　However,　there　is　no　consensus　regarding　whether　aftershocks　must　be　considered　in　the　design　and　retrofitting　of　URM　structures.　This　study　aimed　to　comprehensively　evaluate　the　seismic　performance　and　explore　the　collapse　mechanism　of　URM　structures　subjected　to　sequential　ground　motions.　Therefore,　a　refined　numerical　model　of　a　5-story　URM　structure　was　established　and　verified　using　experiments.　Fifteen　as-recorded　sequential　ground　motions　were　screened　from　several　databases.　By　employing　the　incremental　dynamic　analysis　approach　for　mainshocks　and　aftershocks　separately,　900　samples　of　nonlinear　structural　responses　under　different　sequential　ground　motions　were　obtained.　The　inter-story　drift　ratio　(IDR)　and　damage　index　(DI)　were　adopted　to　assess　the　vulnerability　of　the　URM　structure.　Moreover,　the　entire　process　from　the　cracking　to　collapse　of　a　URM　structure　was　simulated　to　investigate　the　collapse　mechanism　caused　by　aftershocks.　The　results　indicated　that　a　strong　aftershock　poses　a　significant　threat　to　the　URM　structure,　and　the　probability　of　severe　damage　and　collapse　increases　significantly.　The　influence　of　strong　aftershocks　on　the　URM　structure　is　more　significant　in　the　DI-based　vulnerability　assessment　than　in　the　IDR-based　one.　A　strong　aftershock　significantly　aggravates　the　damage　to　parts　previously　damaged　by　the　mainshock　and　has　less　impact　on　the　undamaged　parts.　We　conclude　that　a　strong　aftershock　should　be　considered　in　the　design　and　retrofitting　of　URM　structures.