The residual aluminium was seldomly determined in drinking water treated by polyaluminium chloride(PAC). With the widespread application of polyaluminium chloride to water coagulation,it was possible that the overintake of aluminium could result in some adverse consequences for human and their living environment. Based on themechanism of water coagulativone of PAC,the compound formation of aluminium existing in drinking water,the regularity of the variation of the levels of residual aluminium with the pH value and its relative environmental and biological effects were introduced in details. The relative factors effecting the levels of residual aluminium in drinkingwater and the objective necessity of closely monitoring the variation of the residual aluminium in drinking water wereanalysed in the article.