Kinetics of anti‐solvent crystallization of HNIW polymorphs were studied using in situ Raman spectroscopy and focused beam reflectance measurement (FBRM). Crystal polymorphs and supersaturation were monitored in situ by Raman spectroscopy. Crystal counts and crystal size were monitored by FBRM during anti‐solvent crystallization. The addition rate of the anti‐solvent affected the polymorph transformation, the first polymorph of the crystals, and supersaturation. At a wide range of addition rate, the unstable form, β‐form, was detected and then transformed to the stable form, ϵ‐form. By controlling the concentration and rate of addition, the stable form, ϵ‐form was only nucleated and grown without transformation. This process depended on the supersaturation and affected polymorphic transformation, concentration, nucleation rate, and growth rate, which were simultaneously monitored by Raman spectroscopy and FBRM.