The aim of this study was to investigate the mixing effect on the stoichiometric diversity of benzoic acid-sodium benzoate (HBz-NaBz) cocrystals. The crystallization of HBz-NaBz cocrystals in a 500 mL sized glass vessel was monitored under different agitation speeds and feeding rates of HCl aqueous solution. Under good micromixing and macromixing, the HBz crystals, 2:1 and 1:1 cocrystals of HBz-NaBz were crystallized out rapidly, and all crystals were transformed to a mixture of 2:1 and 1:1 cocrystals of HBz-NaBz in a relatively short amount of time. However, the crystallization of 1:1 cocrystals of HBz-NaBz was delayed by poor micro-, meso-, and macromixing simultaneously. The compositions of the products were altered in different mixing conditions even given the identical experiment time. It was feasible to harvest pure 1:1 or pure 2:1 cocrystals of HBz-NaBz by reaction crystallization through the control of the mixing condition, concentration of reactants, and experiment time. The ternary phase diagram was also constructed for the HBz-NaBz-water system.