An economical and feasible route was used to synthesize the fly ash-based mesoporous CdS/Al-MCM-41 nanocomposites via an alkali fusion step to extract of the silicon and aluminum sources from fly ash, and a templating step to assemble nanocomposites at room temperature. The low angle X-ray diffraction and high resolution transmission electron microscopy results indicated that the mesoporous Al-MCM-41 was formed with average pore size of about 3.0 nm and the CdS clusters were assembled in the channels of Al-MCM-41. The results of ultraviolet-visible (Vis) diffuse reflectance spectra and the fluorescence emission spectra revealed that the nanocomposites show a strong absorption edge at 521 nm and a weak photoluminescence peak at 398 nm. The activities of hydrogen production were evaluated by photocatalytic water-splitting under Vis light irradiation, and the CdS/Al-MCM-41 nanocomposites showed the highest H-2 evolution in amount of 3.3 mL/g during the reaction time for 6 h due to the synergistic effect between CdS clusters and mesoporous Al-MCM-41 matrix. A mechanism of photocatalytic H-2 production was proposed.