Mg-Zn-Er　casting　magnesium　alloys　have　good　properties,　such　as　high　specific　strength,　high　specific　stiffness　and　remarkable　temperature　creep　properties.　Current　researches　mainly　focused　on　the　phases　and　mechanical　properties　at　room　and　high　temperatures.　However,　the　effect　of　Er　on　hot　cracking　susceptibility　of　Mg-5Zn-xEr　magnesium　alloys　was　barely　studied.　In　this　work,　a　modified　RDG　(Rappaz-Drezet-Gremaud)　model　for　predicting　the　hot　cracking　susceptibility　of　Mg-5Zn-xEr　(x=0.83,　1.25,　2.5,　5,　mass　fraction,　%)　ternary　alloys　was　proposed,　which　considered　the　effects　of　phase　and　solidification　temperature　range　on　the　hot　cracking　susceptibility　of　the　multiphase　alloys.　And,　the　hot　cracking　susceptibility　was　evaluated　by　the　experiment　of　constrained　rod　casting　(CRC).　The　results　indicated　that　the　modified　RDG　model　could　accurately　predict　the　hot　cracking　susceptibility　of　Mg-5Zn-xEr　magnesium　alloys.　The　hot　cracking　susceptibility　increased　with　the　addition　of　Er　up　to　2.5%,　and　Mg-5Zn-2.5Er　alloy　showed　the　maximal　hot　cracking　susceptibility;　when　the　addition　of　Er　increased　to　5.0%,　Mg-5Zn-5Er　alloy　exhibited　the　minimal　hot　cracking　susceptibility.　The　calculated　results　were　consistent　with　the　experimental　ones.　Further　analysis　on　the　casting　solidification　curves,　phases　and　microstructures　showed　that　I-phase　precipitated　by　peritectic　reaction　during　solidification　of　Mg-5Zn-2.5Er　alloy　depleted　liquid　phases　and　extended　the　solidification　temperature　range　of　the　alloy,　leading　to　the　hot　cracking　susceptibility　increasing.　The　Mg-5Zn-5Er　alloy　underwent　eutectic　reaction　of　L　-＞alpha-Mg+W　during　solidification,　which　reduced　the　solidification　temperature　range.　Meanwhile,　this　process　was　beneficial　to　feeding　the　interdendritic　hot　cracking　in　the　terminal　period　of　solidification,　which　significantly　decreased　the　hot　cracking　susceptibility　of　Mg-5Zn-5Er　alloy.