The semi-solid billet of AlMgLi0.5Zn0.5Cu0.2 light-high entropy alloy was prepared by isothermal heat treatment and the effects of holding temperature and holding time on the microstructure evolution of AlMgLi0.5Zn0.5Cu0.2 light-high entropy alloy were investigated. The results show that, with the increase of holding temperature, the average grain size of semi-solid AlMgLi0.5Zn0.5Cu0.2 light-weight high entropy alloy 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 AlMgLi0.5Zn0.5Cu0.2 light-weight high entropy alloy increase. The best semi-solid microstructure of AlMgLi0.5Zn0.5Cu0.2 light-weight high entropy alloy was obatined at 460℃ for 60min, whose average grain size is about 41μm, roundness is 0.82, and coarsest coefficient K=20.95μm³/s.