Al15Mg45Li39Ca0.5Si0.5 LW-HEAs with a density of 1.45 g/cm3 was designed and fabricated by magnetic suspension furnace. The non-dendritic globular microstructures of Al15Mg45Li39Ca0.5Si0.5LW-HEAs were prepared by isothermal heat treatment,and the effects of isothermal temperatures and holding times on the microstructures of semi-solid Al15Mg45Li39Ca0.5Si0.5LW-HEAs were investigated by optical microscope and scanning electron microscope. The results indicate that the phasesof Al15Mg45Li39Ca0.5Si0.5LW-HEAs consist of HCP phases and intermetallic compounds. With the increase of isothermaltemperature, the average grain size increases and the shape factor increases first and then decreases. With the prolongationof holding time, the average grain size increases first, then decreases and then increases again, and the shape factor alsoincreases first and then decreases. The semi-solid Al15Mg45Li39Ca0.5Si0.5LW-HEAs have a low coarsening coefficient. Compared with the as-cast Al15Mg45Li39Ca0.5Si0.5LW-HEAs, the ultimate strength, plasicity and hardness of smei-solidAl15Mg45Li39Ca0.5Si0.5LW-HEAs are improved by 25.7%, 18.5% and 89.7%, respectively.