A series of plasma characteristic spectral lines of Mg alloy were obtained under nanosecond laser shock produced by a pulsed Nd∶YAG laser (1 064 nm, maximum energy 500 mJ), which was taken under standard atmospheric pressure and at room temperature. Results indicated that the evolutionary rates of spectral lines were different, and the laser energy was enough to ionization Mg alloy under this experimental condition by the spectral lines of MgⅠ, MgⅡ. The electron temperature of Mg plasma were calculated by the measured relative Emission-line intensity（MgⅠ383.2 nm, MgⅠ470.3 nm, MgⅠ518.4 nm）. The experimental results showed that the secondary excitation Mg atoms could be got under this experimental condition. The electron temperature of Mg plasma decreased with the laser energy reduced while the laser energy was in the range of 200～500 mJ. When the laser energy was in the range of 350～500 mJ, the electron temperature changed rapidly. When the laser energy was in the range of 200～350 mJ, the electron temperature changed slowly and tended towards stability. It is found that when the laser energy was 300 mJ, the plasma temperature raised suddenly, which could not accord with the trend because of plasma shielding. When the laser energy was 300 mJ, the relative intensity of spectral lines was reduced which was lower than that of 350 and 250 mJ. And it is against the variation trend of the relative intensity of spectral lines increases with the increase of laser energy, which prove plasma shielding phenomenon had occurred and plasma with high power laser separate the coupling between laser and material. The plasma temperature was increased significantly, which is not consistent with the trend .When the plasma shielding phenomenon happened, laser energy was absorbed by the plasma, resulting in the rise of plasma temperature.