Micro-nano bubble flooding is an emerging enhanced oil recovery (EOR) technique. However, its phase behavior, stability, and oil displacement mechanisms remain unclear under high-temperature, high-pressure (HTHP) conditions, especially in the supercritical state. In this study, an improved fluid particle analysis system was adopted to achieve in-situ characterization of the size and concentration of micro-nano bubbles at HTHP conditions. On the basis of clarifying the evolution laws of the size and concentration of micro-nano CO? dispersed phases under varying temperatures and pressures, the morphological stability of micro-nano CO? dispersed phases was evaluated under variable temperature-pressure conditions. Core flooding experiments of micro-nano CO? in shale were conducted, combined with the analysis of concentration variation, dynamic instability index, and CO? gas release volume during the displacement process, to reveal the phase transition characteristics and oil displacement mechanisms of the micro-nano CO? system. Results show that the average diameter of the laboratory-prepared micro-nano CO? system ranges from 223 nm to 386 nm, exhibiting a typical micro-nano scale distribution. When the pressure exceeds the critical pressure of CO?, the dispersed phase transforms into a supercritical fluid state, while the system still maintains a two-phase structure with the aqueous phase as the continuous phase and supercritical CO? as the micro-nano dispersed phase. The volume expansion rate of the micro-nano CO? dispersed phase gradually slows down with the increase of temperature and pressure, whereas the concentration decay rate shows no obvious correlation with temperature and pressure. During shale oil displacement, the micro-nano CO? dispersed phase gradually converts into CO? gas at a constant concentration decay rate, endowing it with stronger pore penetration capacity and forming a dual synergistic oil displacement mechanism of "micro-nano dispersed phase flooding + gas flooding". The cumulative oil recovery reaches 43.4%, which is 15.6 percentage points higher than that of the bubble-free base fluid. Reducing the average diameter and increasing the concentration of the CO? dispersed phase contributes to an improvement in oil displacement efficiency by 2.4 to 6.4 percentage points. This study verifies that the micro-nano CO? system still possesses favorable stability and oil displacement performance under supercritical conditions, which can provide theoretical and technical support for the efficient development of shale oil.