High-pressure microfluidic systems exhibit favorable capacity to enhance mixing quality compared to conventional macroscale liquid systems. These mixing conditions are very interesting for the preparation of fluorescent organic nanoparticles by supercritical antisolvent process. In this work, fundamental mechanisms of microfluidic supercritical antisolvent process (µSAS) including thermodynamics, hydrodynamics, nucleation/growth phenomena, are investigated using a coupled experimental/simulation approach. Specifically, we determined experimentally the particle precipitation field in a CO2 / solvent medium using a fluorescent organic dye molecule which presents an enhancement of fluorescence intensity in its aggregated state (AIE effect). The results of the direct numerical simulation considering all the physical phenomena are compared with the experimental data for validation and deep understanding of the mechanisms. It is shown that despite ultra-short mixing time, the supersaturation field showed some fluctuation leading to variation of the nucleation times.