A new technology combined with slightly electromagnetic stirring and melt isothermal holding technology was formed: Two-way slightly electromagnetic stirring and isothermal holding. The effects of the isothermal holding temperature from 580℃ to 610 ℃and the isothermal holding time from 3min to 15min on the evolution of solidification microstructure were researched. The results indicated that under different isothermal heat treatment parameters, as the temperature increased and time prolonged, the size of the primary grains was gradually refined and spheroidized. However, the threshold value of the isothermal holding temperature and time could be found, so the grains became coarsened. Above the threshold value, by comparing the microstructure of the grain, it was found that the proper parameters were the isothermal holding temperature of 600℃and the isothermal holding time under 7min, at this conditions, the average equal-area circle diameter and average shape factor of the primary phase were 29.4μm and 0.86, respectively. Therefore, the proper temperature and time was the semisolid A356 aluminum alloy in the composite process to match the reasonable melt isothermal holding process parameters. The qualified semisolid aluminum alloy slurry could be prepared by the new composite process.