This paper investigates the factors leading to non-linear relationships in the low concentration region of photometrio method and the means by which these drawbacks can be overcome. Al-zylenol orange system is chosen for investigation. By the study of absorption spectra and isoabestio points of the system under various conditions, it is found that when its pH changes gradually (the concentrations of Al and zylenol orange are kept constant) or when the concentration of Al increases gradually (keeping the concentration of zylenol orange and pH unchanged), there are two isoabestio points appearing in the families of curves of the system. From the analyses of is}bestio points, it is ascertained that isoabestio points appear only when the system forms oomplezes to one reaction scheme and is not related in any way to other colored compounds involved in this reaction. No isosbestio points appear when oompleging is taken place according to two reaction schemes simultaneously. Therefore,if the determination of Al is carried out with this system, the range of variation of Al concentration must be limited within the region of egistanoe of sosbestio points in order to keep linear relationship, otherwise nonlinearity is obtained.For the sake of overcoming the phenomenon of nonlinearity in the ordinary method of Al determination, this paper adopts a method of preliminary addition of appropriate quantity of Al into gylenol orange as color reagent, so that the reaction will strictly proceed according to one reaction scheme. The results then will be straight lines.