The analysis of aluminium (Al) in the seaweed is usually according to GB/T 23374-2009 which is in fact the method for measuring total Al. However, most of the Al in the seaweed exists in organic forms which can not be absorbed by biological organism, and the toxic soluble ionic Al (Ⅲ) is very low. So it is urgent to establish a method for the Al speciation analysis in the seaweed. In the present research, the liquid Chromatography-Inductively Coupled Plasma Mass Spectrometry (HPLC-ICP-MS) was used to determine the Al (Ⅲ) in the seaweed. The samples were extracted by the eluent (0.040mol/L LiOH,0.006 mol/L PDCA and 0.090 mol/L CH3COOH) and shaken under 70 ℃ for 2 h, and then the extract solution was centrifuged and filtered. After separation by CS5A column, the Al (Ⅲ) was tested by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The concentration of standard Al solutiors (Ⅲ) were 5.0mg/kg and50.0mg/kg, and the average recoveries were all above 90.8%, and the relative standard deviations were all below 4% (n=5). This method was successfully used in the analysis of real samples and proved to be very accurate and reliable, which may provide important technical support for drawing up the agricultural professional standard.