A ZnGa₂O₄:Cr³⁺-based aerogel was prepared by using polyacrylic acid (PAA) as a dispersant and propylene oxide (PO) as a crosslinking agent via CO₂ supercritical drying. The results of BET and SEM show that there is a certain degree of macroporosity (d > 50 nm) in the aerogel. It has a dendritic structure and the interior is relatively loose. EDS mapping illustrates that the elements Zn, Ga, and Cr are evenly distributed in the aerogel. In addition, the diffuse reflectance spectra and the emission spectrum of samples with different calcination temperatures were also characterized. Both demonstrated, when the calcination temperature is greater than 600 °C, that the sample crystallizes and has a significant emission, which is consistent with the XRD and TG-DSG results. Finally, the ZnGa₂O₄:Cr³⁺-based aerogel also exhibits excellent long afterglow performance and high photocatalytic performance with 80.1% methylene blue (MB) degradation at 20 min.