Using aluminum alloy 6061-T6 as the substrate and sulfuric acid (0.3 M) anodizing technology, we formed nanoporous anodic aluminum oxide (NPAAO) films for solar absorption and photothermal sensor applications in a medium temperature range. We found that the diameter and thickness of aluminum oxide increased with increasing applied anodic voltage. The increase in the applied anodic voltage, which increased the NPAAO film thickness, enhanced absorption over the solar spectral range. Therefore, the solar absorptivity increased with the thickness of the NPAAO film, and more of the penetrating light was trapped by the thicker NPAAO film due to its greater depth and higher pore density. Moreover, the transient temperature of the NPAAO films was measured under 500 W halogen light illumination to investigate the photothermal efficiency of the films. The results indicated that the steady-state temperature increased with the absorptivity of the NPAAO films. The mean reflectance of the aluminum alloy specimens in the spectral range of 250–850 nm was greater than 72% before the anodizing oxidation process and less than 7.7% after the process. Therefore, the applied anodic voltage, which changes the pore density and the thickness of the anodized film, is an important factor determining the solar absorption performance of NPAAO films.