Ferron colorimetric method has been widely used in aluminum speciation analysis. However, some important factors that may affect the accurate determination of mononuclear Al concentration have not been studied yet. Using molar absorption coefficient ε as a key indicator, the critical issues in Al-Ferron coloring reaction system have been investigated experimentally in this paper, including different Ferron system, Ferron solubility, experimental temperature, ionic strength and the reference system. The results show that three vital factors (different Ferron colorimetric system, Ferron solubility and experimental temperature) would significantly affect the accurate measurement of mononuclear Al concentration; whereas, other two factors like ionic strength and the reference have litter influence. Base on our experimental results and statistic analysis of relevant important literatures, we suggest that optimizing colorimetric conditions and standardizing operational procedures should be paid much more attention in Ferron colorimetric assay in order to obtain the accurate measurement results.