Supercritical CO₂ exhibits lower viscosity and higher diffusivity than gaseous CO₂. It can rapidly penetrate the cement matrix, leading to notable pore compaction, reaction efficiency improvement, and modification effect enhancement. However, there are almost no studies on recycled coarse aggregate (RCA) modified by supercritical accelerated carbonation. In this paper, an exploratory study was attempted in which the RCA samples were prepared for supercritical carbonation modification experiments. The influences of initial moisture content (IMC) and aggregate particle diameter (APD) of RCA on supercritical accelerated carbonation improvement were preliminarily revealed. The interaction mode between the internal pore water of RCA samples and supercritical CO₂ was clarified. This study indicates that the degrees of improvement for the apparent density, water absorption, and actual mass increase of RCA samples after supercritical carbonation all show a trend of first increasing and then decreasing with the variation in the IMC of RCA. The RCA samples exhibit the best improved properties after only 3 h of supercritical carbonation when the IMC consistent with the natural-state RCA is matched to an APD of 10–20 mm, which indicates that rapid carbonation and high-efficiency modification of RCA can be achieved based on supercritical CO₂.