In　addition　to　the　three　well-known　Ag-related　precipitates　(Ω,　X′　and　Z)　in　the　Al-Cu-Mg-Ag　alloys,　Ag　can　also　be　involved　in　the　formation　of　the　as-cast　second　phases.　However,　the　effect　of　Ag　addition　in　Al-Cu-Mg-Ag　alloys　has　not　been　completely　studied　and　even　the　structure　of　the　as-cast　Ag-containing　phases　is　still　controversial.　By　employing　the　focused　ion　beam　(FIB)　combined　with　transmission　electron　microscopy　(TEM)　techniques　and　density　functional　theory　(DFT)　calculations,　the　formation　mechanisms　of　the　Ag-containing　phases　in　the　as-cast　Al-Cu-Mg-Ag　alloys　have　been　investigated.　The　Ag-containing　phases　are　a　series　of　hexagonal　C14-type　Laves　phases　with　continuously　varying　Ag　concentrations,　described　as　(AlxCuyAg1-x-y)2Mg.　Moreover,　the　specific　occupancy　sites　of　the　atoms　in　(AlxCuyAg1-x-y)2Mg　were　determined.　The　formation　of　the　(AlxCuyAg1-x-y)2Mg　can　be　attributed　to　the　stronger　Ag-induced　aggregation　of　solute　atoms　in　the　initial　stage　and　the　establishment　of　strong　Ag-X　(X　=　Al,　Mg　and　Ag)　bonding　in　the　Ag-containing　phases.　Furthermore,　our　experiments　have　revealed　the　solidification　sequence　of　Al-Cu-Mg-Ag　alloys,　and　pointed　out　that　(AlxCuyAg1-x-y)2Mg　is　formed　at　a　lower　temperature　(493.9　°C)　through　the　reaction　L　⇌　Al2CuMg　+　(AlxCuyAg1-x-y)2Mg.　The　study　could　have　positive　implications　for　refinement　of　the　Al-Cu-Mg-Ag　quaternary　phase　diagram　and　promote　the　composition-property　design　of　novel　aluminum　alloys　based　on　(AlxCuyAg1-x-y)2Mg　in　the　future.　©　2023