To clearly understand the crystallization process of organic micro/nano crystalline (OMC) materials in solution is a long-standing challenge because of the difficulty in the separation of intermediates and in situ and real-time monitoring the process. Herein, we report the exploration for the crystallization process of OMC materials from the amorphous intermediates by taking the advantage of the spectral change of an environment-sensitive emission dye BF2bcz. The intermediate for the formation of the OMC materials by solvent-exchange method was separated as amorphous nanospheres which were transformed into crystalline nanorods by adding the surfactant to their aqueous dispersion. The distinct emission properties of the amorphous molecular aggregates and nanorods, used as fingerprint for each species, allow for in situ and real-time monitoring the crystallization process by using fluorescence spectroscopy. Such facile method readily identified that increasing the concentrations of surfactant and temperatures both accelerated the crystallization process of BF2bcz in aqueous solution, while the size of the nanorods increased with the decrease of concentrations of surfactant. Our work provided direct experimental evidence to support the two-step nucleation mechanism in the preparation of OMC by solvent-exchange method.