Herein, we describe a facile method to control the size and dispersion of CuO nanoparticles (NPs) in SBA-15 by the supercritical antisolvent (SAS) method during the depressurization process of supercritical fluid deposition (SCFD), describing the mass transfer between supercritical carbon dioxide (scCO₂) and a cosolvent. On reducing the concentration of the cupric nitrate precursor, the size of CuO NPs was decreased from 3.5 ± 1.0 to 1.0 ± 0.4 nm (Cu loading 14.2–12.1 wt %) under the depressurized rate of ∼20 MPa/min. Based on the size distribution and nucleation theory, the nucleation rate of the precursor could reach 10²⁵ cm–³·s–¹, and the calculated interface energy of cupric nitrate was less than 0.1 J/m², indicating that scCO₂ as an antisolvent caused the heterogeneous nucleation of the precursor on the surface of SBA-15 channels. Hence, the nucleation rate and growth of cupric nitrate were regulated under a given scCO₂ depressurized rate, and then the CuO NPs’ size distribution and Cu loading were controlled subsequent to calcination.