Supercritical fluid anti-solvent precipitation has been attracting widespread attention due to its distinctive advantages, and has exhibited a great perspective of application in the production of polymer-based composite micro- and nanoparticles. In this study, based on the experiment results of production of carotene-loaded polymer PEG or l-PLA composite microparticles using solution enhanced dispersion by supercritical fluids through prefilming atomization (SEDS-PA) process, the possible mechanism of dispersing an active component in a polymeric carrier by the SEDS-PA co-precipitation was deduced. The mechanism is mainly the formation and growth of the active component (carotene) nuclei in the polymer-rich phase induced by mass transfer and phase transition, and the polymer capture/encapsulation of active component particles generated in an expanded solution droplet caused by the collision among these particles and polymer-rich phase. There are four factors that could influence the sizes and morphologies of the SEDS-PA precipitates. They are, respectively, atomization of solution, prompt and persistent super-saturation of the expanded droplets, breakup of the expanded droplets with interstices and the particle agglomeration caused by collision in the SEDS-PA process. The integrated effect of these factors dominates the sizes and morphologies of the SEDS precipitates.