Herein, non-toxic, ample, and cheap water served as the solvent to dissolve Cu and Mn nitrate, chelating agent, and wetting agent, where a long-term stable sol solution could be easily obtained. The single layer solar absorber film derived from the above precursor has been successfully deposited on a stainless steel substrate via a facile and low-cost sol–gel dip-coating method. A spinel Cu_1.5Mn_1.5O₄ phase for the single layer thin film could be achieved at the annealing temperature of 450 °C for 1 h. The effect of annealing temperature on the surface structure and optical properties of the thin film was systematically studied. The results revealed that the film thickness, integration of impurities, and surface texture (crystallinity, nanoporosity, and roughness) coordinately influenced solar absorptance and thermal emittance of the thin film. A Cu_1.5Mn_1.5O₄ single layer thin film annealed at 500 °C exhibited excellent solar absorptance (α_s = 0.914) and thermal emittance (ε₁₀₀ = 0.125). Furthermore, the single layer spinel thin film also showed benign thermal stability at the corresponding working temperature. Consequently, it could be used as a promising candidate for solar absorber thin films and safely applied in low-to-mid temperature regions.