The kinetics of precipitation in Al—0.6Mg—0.9Si—0.2Cu (mass fraction, %) alloy under different conditions was investigated by differential scanning calorimetric analyses (DSC) and microhardness measurements. The results show that, an exothermic peak at about 100—-150℃ can be observed in the DSC curve for the nature aged (T4) alloy, but no peak for the pre—aged (T4P) alloy, and the peak corresponding with β″ phase moves toward the low temperature zone for T4P alloy. The fraction of transformation Y, the rate of transformation dY/dT, and the kinetic parameters such as activation energy Q and frequency factor k₀ for GP zones dissolution and β″phase transformation were calculated by Avrami—Johnson—Mehl equation. The values of activation energy for GP zones dissolution after natural aging and pre—aging were 66 and 119 kJ/mol, respectively, and the values forβ″phase formation after natural aging and pre—aging were 114 and 60 kJ/mol, respectively. The kinetics expressions were obtained as follows, for T4 alloy:Y_GP=1-exp[-1.3×10⁷t exp(-7977/T)],Y_β″=1-exp[-4.7×10²²t² exp(-27484/T)];for T4P alloy: Y_GP=1-exp[-2.4×10¹³ t exp(-14345/T)],Y_β″=1-exp[-2.9×10¹¹t² exp(-14392/T)].In addition, with the increasing of aging time, the wholetrend of hardness changing for pre—aged alloy is increasing at first, and then keeps constant basically,but for the nature aged alloy, the hardness decreases after aging at 185℃ for 20 min, which was explained by the kinetics obtained above.