Microstructure evolution of extruded Mg-2Zn-xY (x=0.5, 1, 2, at%) alloys in semi-solid isothermal heat treatment process was investigated. The results show that in these three kinds of Mg-2Zn-xY alloys, α-Mg grain size of extruded Mg-2Zn-1Y alloy is the minimum, and a large number of twins are found in extruded Mg-2Zn-2Y alloy. When the semi-solid isothermal time is 10 min, with increasing temperature, α-Mg solid phase grains in Mg-2Zn-xY alloy are gradually spheroidized, and separated by liquid phases. Otherwise, the liquid phase contents located both at the grain boundaries and in the grain interior significantly increase. With the increase of isothermal time (≤30 min) at 580 °C, Ostwald ripening mechanism acts a major role during microstructure evolution of semi-solid Mg-2Zn-0.5Y alloy. However, both Ostwald ripening mechanism and solid particle melting mechanism act major roles at the same time during microstructure evolution of semi-solid Mg-2Zn-1Y and Mg-2Zn-2Y alloys