Many semiconductors with broad solar spectrum absorption have always been pursuing for the effective photothermal conversion process. However, it is still a challenge to modulate the structural and bandgap of semiconductors to enhance the photothermal activity synergistically. In this work, facile prepared mesoporous CuO displays a narrower bandgap than non-porous CuO, leading to highly efficient solar-thermal conversion. Mesoporous CuO shows obvious broad and strong optical absorption, especially in the visible light region. It is noteworthy that the mesoporous CuO nanofluids exhibit good dispersibility in water. Different concentrations of mesoporous CuO nanofluids have higher temperature rise than non-porous CuO nanofluids. The photothermal conversion efficiency of 50 ppm mesoporous CuO/water nanofluid is 83.66%, compared with 58.86% for 50 ppm CuO/water nanofluid. Mesoporous CuO will bring a new paradigm for mesoporous metal oxide nanofluids as working fluids in direct absorption solar collectors.