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The transformation behavior of secondary phases in conventional ingot and ultrasonic ingot during heat-treatment process were investigated by OM, SEM, EDS and DSC. Crystalline phase transition was observed by SEM with <i>in situ</i> heating station. The experiment results indicate that the ultrasonic melt treatment transforms from columnar crystals to equiaxed crystals, and obtains fine and uniform grains. The melting enthalpy of ultrasonic ingot is 4.60J/g, while the melting enthalpy of conventional ingot is 6.52J/g, combined with EDS analysis results, θ phase and T phase are found to be reduced by 29% after ultrasonic melt treatment. The secondary phases in the ingots include θ phase, T phase, Al₃Fe phase and Mg₂Si phase. θ phase is dissolved into matrix gradually, T phase transforms to S phase and is dissolved into matrix at high peak temperature, Al₃Fe phase transforms to Al₇Cu₂Fe phase at low peak temperature and remains in the ingots, Mg₂Si phase changes nothing during DSC analysis process.