A novel simple strategy for alkylamine-directed self-assembly of Weakley-type polyoxometalate (POM, Na9[EuW10O36]·32H2O, abbreviated to EuW10) to form three-dimensional nanoflowers has been successfully developed through the ionic self-assembly (ISA) method. For comparison, different molecular weights of alkylamines including diethylenetriamine, triethylenetetramine, and tetraethylenepentamine (TEPA) were selected to construct hierarchical nanostructures. Our results revealed that the morphologies and sizes of the nanostructures could be simply controlled by varying the molecular weights and concentrations of alkylamines. The fluorescent color of EuW10/TEPA nanoflowers changed compared with that of EuW10 owing to the varied symmetry degree of europium coordination in EuW10/TEPA nanoflowers. It is demonstrated that this effective self-assembly occurs mainly though the hydrogen bond and electrostatic interaction between EuW10 and TEPA. What's more, the EuW10/TEPA nanoflowers after calcining showed excellent decomposition efficiency toward methylene blue dyes. Our results further confirmed that ISA method between small molecules and POM can provide a unique "bottom-up" strategy to construct novel structures with functional properties.