The supercritical fluid region is typically considered an unbalanced “phase” region, making it difficult to quantitatively analyze and explain the transition of the phase interface. In this chapter, we propose and implement a phase-field method with temperature as the order parameter, coupled with a free-energy minimization method under fixed initial pressure and density conditions, to predict the continuous phase evolution of supercritical CO2. Based on the continuous phase transition of supercritical CO2, the density distribution is predicted using a linear approximation of the temperature under isobaric conditions. In this method, the Peng-Robinson equation is employed to calculate the free energy function of supercritical CO2, and temperature is selected as the phase-field parameter in the Cahn–Hilliard equation to analyze the density map, regional transient statistical behavior, and local density fluctuation profiles under different operating conditions. The results showed that the fast separation of density distribution into “pseudo-liquid islands” and other “pseudo-gas regions” mainly happens within the 0.3 s. With the increase of time, the inner density of the cavity changes constantly. The average density of Case 1 is finally around at 600 kg/m3, and the maximum difference of local density is 40 kg/m3.