To monitor the components of molten magnesium alloy during the smelting process in real time and online, we designed a standoff double-pulse laser-induced breakdown spectroscopy (LIBS) analysis system that can perform focusing, collecting and imaging of long-range samples. First, we tested the system on solid standard magnesium alloy samples in the laboratory to establish a basis for the online monitoring of the components of molten magnesium alloy in the future. The experimental results show that the diameters of the focus spots are approximately 1 mm at a range of 3 m, the ablation depth of the double-pulse mode is much deeper than that of the single-pulse mode, the optimum interpulse delay of the double pulse is inconsistent at different ranges, and the spectral intensity decays rapidly as the range increases. In addition, the enhancement effect of the double pulse at 1.89 m is greater than that at 2.97 m, the maximum enhancement is 7.1-fold for the Y(I)550.35-nm line at 1.89 m, and the calibration results at 1.89 m are better than those at 2.97 m. At 1.89 m, the determination coefficients (R2) of the calibration curves are approximately 99% for Y, Pr, and Zr; the relative standard deviations (RSDs) are less than 10% for Y, Pr, and Zr; the root mean square errors (RMSEs) are less than 0.037% for Pr and Zr; the limits of detection (LODs) are less than 1000 ppm for Y, Pr, and Zr; and the LODs of Y, Pr, and Zr at 2.97 m are higher than those at 1.89 m. Additionally, we tested the system on molten magnesium alloy in a magnesium alloy plant. The calibration results of the liquid magnesium alloy are not as favorable as those of the sampling solid magnesium alloys. In particular, the RSDs of the liquid magnesium alloy are approximately 20% for Pr and La. However, with future improvements in the experimental conditions, the developed system is promising for the in situ analysis of molten magnesium alloy.