Rectangular-tube-based fan-like polyaniline has been readily fabricated by the oxidation polymerization of low-concentration aniline in dilute HCl aqueous solution for the first time. Different from the conventional polymerization that proceeds in concentrated acidic solution (to produce nanofibers), this synthesis was conducted in dilute inorganic acid solution at room temperature and created polyaniline hierarchical fan-like architectures. The morphological evolution of the product has been followed with respect to the reaction time, based on which conceivable interpretations of the formation of rectangular tubes and their assembly into fan-like architectures are given. Other inorganic acids (HNO3, H2SO4, and H3PO4) with low concentrations have been used to replace HCl in the reaction system to validate the adaptability of this approach in this report.