Transition metal borides are a kind of potential materials for high-temperature solar thermal applications. In this work, a novel SS/HfB2/Al2O3 tandem absorber was prepared, which exhibited high solar spectrum selectivity (a/3) of 0.920/0.109. The optical constants of the coating were obtained using spectroscopic ellipsometry, and the dispersion model of the HfB2 layer was modeled with the Tauc–Lorentz dispersion formula. In addition, the reflectance spectrum simulated by the CODE software corroborated well with the experimental results. The thermal stability test indicated that the HfB2/Al2O3 solar absorber coating was thermally stable in vacuum at 600 C for 2 h. When extending the annealing time to 100 h, the coating could maintain high spectral selectivity after aging at 500 C irrespective of whether in air or vacuum. All these results indicate that the coating has good solar selectivity and is a promising candidate for high-temperature solar thermal applications