The appearance of concomitant crystals will affect the quality of the crystalline pharmaceutical products. Although many researchers have investigated the concomitant polymorphs, rather little attention has been paid to the solvate systems, which is also a very important solid formation in the pharmaceutical industry. In this study, the concomitant crystallization of pure cefuroxime acid and its acetonitrile solvate was found in the binary mixture of acetonitrile and water. The concomitant crystals were confirmed and verified by a series of off-line and online techniques, including optical microscopy, powder X-ray diffraction, DSC, and Raman spectra. Furthermore, this concomitant crystallization phenomenon was explained with the discontinuity point in the solubility curve of pure cefuroxime acid. Finally, the nucleation and growth mechanisms of the concomitant crystals were identified by fitting the induction period data.