LIBS on-line analyzer for liquid metal composition, which is designed by the LIBS project group of Shenyang Institute of automation, Chinese Academy of Sciences, is a analytical system for on-line monitoring of molten metal components in the metallurgical industry. We used this system to monitor the composition of molten aluminum in melting and casting plant of Liaoning Zhongwang Group Co., Ltd. After several months testing, we obtained that the optimum flow rate of argon in the probe tube is 1~1.4 L/min, and when the detection depth is located at the position where the pressure fluctuation range of argon is ±300 Pa and the pressure fluctuation is regular, the concentration of each element is stable. Finally, we calibrated our equipment by solid sampling data from laboratory analysis. The obtained results of on-line monitoring are as follows: the repeatability accuracy of Si、Fe、Cu、Mn and Cr is less than 2%; the measurement deviationis less than 0.01% for Si, Fe, Mg (concentration is between 0.1%~1%), and is less than 0.001% for Cu, Mn and Cr (concentration is less than 0.1%); the stability of the multiple rise and fall of the probe is less than 3%. On-line monitoring data meet the requirements of factory, so we can realize real time and on-line monitoring the compositions of molten aluminum.