The microstructure evolution and phase transformation of Mg-16Zn-5Al alloy were investigated during partial remelting. Results show that when remelting parameter is 440℃+(30~60min) or 460℃+30min, Mg-16Zn-5Al alloy can obtain the idea semi-solid spherical microstructure. The average grain size is 54.9~57.9μm, and the average roundness is 1.18~1.29, and the solid fraction is about 65%. The phases transformation which include MgZn+τ→α-Mg, α-Mg+MgZn+τ→L, α-Mg→L, α-Mg→L and L→α-Mg were occurred during partial remelting. They lead to the initial rapid coarsening of the grain, structure separation, spheroidization of the grain and final merger coarsening. Meanwhile, the subboundary and root remelting within the alloy play an important role in separation of microstructure. The coarsening factor Mn which related with diffusion coefficient at a isothermal period was deduced by LSW theory. The main evolution way of grain is described by Mn at corresponding period.