The effects of isothermal heat treatment on the semi-solid microstructure evolution of VW63Z(Mg-6Gd-3Y-0.4Zr,wt.％) alloy are studied. It shows that the microstructure of VW63Z alloy could transform from equiaxed crystal to semi-solid spherical crystal after isothermal heat treatment above 620 oC. With the heating temperature elevating from 620 oC to 635 oC and the holding time prolonging from 10 min to 35 min,the liquid fraction increases gradually. The semi-solid microstructure evolution of VW63Z alloy can be divided into three stages,i. e., particle coarsening and spheroidization;particle necking,coalescence,and Ostwald ripening;and dynamic equilibrium. The semi-solid process window of VW63Z alloy ranges from 620 oC to 635 oC,where the best process parameters are holding at 635 oC for 20 min?30 min. The solid fraction,the average particle size,and the shape factor are 41.1％?53.8％,81.5 μm?83.2 μm,and 0.70?0.75,respectively. The maximum relative deviations of the solid fraction,the particle size,and the shape factor at different heights of the same billet are 44.6％,17.4％,and 16.6％,respectively, which means that it should pay attention to the uniformity of edge and core of VW63Z alloy during isothermal heat treatment. The driving force of microstructure is supposed to be the reduction of solid-liquid interface free energy.