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In this work, polyamidoamine (PAMAM)-functionalized water-stable Al-based metal–organic frameworks (MIL-53(Al)-NH₂) were proposed with enhanced fluorescence intensity, and used for the sensitive detection of heavy metal ions in aqueous solution. The size and morphology of MIL-53(Al)-NH₂ were effectively optimized by regulating the component of the reaction solvents. PAMAM dendrimers were subsequently grafted onto the surface with glutaraldehyde as a cross-linking agent. It was found that the size and morphology of MIL-53(Al)-NH₂ have great influence on their fluorescence properties, and PAMAM grafting could distinctly further improve their fluorescence intensity. With higher fluorescence intensity, the PAMAM-grafted MIL-53(Al)-NH₂ showed good linearity (R² = 0.9925–0.9990) and satisfactory sensitivity (LOD = 1.1–8.6 μmol) in heavy metal ions determination. Fluorescence enhancement and heavy metal ions detection mechanisms were discussed following the experimental results. Furthermore, analogous water-stable Materials of Institute Lavoisier (MIL) metal–organic frameworks such as MIL-53(Fe)-NH₂ were also proved to have similar fluorescence enhancement performance after PAMAM modification, which demonstrates the universality of the method and the great application prospects in the design of PAMAM-functionalized high-sensitivity fluorescence sensors.