The semi-solid billet of light -high entropy alloy AlMgLi_0.5Zn_0.5Cu_0.2 was prepared by isothermal heat treatment and the effects of holding temperature and holding time on the microstructure evolution of the lighthigh entropy alloy AlMgLi_0.5Zn_0.5Cu_0.2 were investigated. The results show that with the increase of holding temperature, the average grain size of semi-solid light-weight high entropy alloy AlMgLi_0.5Zn_0.5Cu_0.2 decreases first and then increases, while the roundness of the grains increases first and then decreases. With the prolongation of holding time, the average grain size and roundness of the light-weight high entropy alloy AlMgLi_0.5Zn_0.5Cu_0.2 increase. The best semi-solid microstructure of the light-weight high entropy alloy AlMgLi_0.5Zn_0.5Cu_0.2 was obtained at 460 ℃ for 60 min, whose average grain size is about 41 μm, roundness is 0.82, and coarsest coefficient K=20.95 μm³ /s.