The coagulation dynamics of acrylic polymer (PAN) with 1-butyl-3-methylimidazolium chloride [BMIM]CI as solvent for PAN and H2O as nonsolvent was investigated in detail. On the basis of Fick's second law of diffusion, a mass-transfer model of [BMIM]CI from concentrated PAN/[BMIM]CI solution was established as verified with the experimental data. The established model has a good fit with the experimental data and the diffusion coefficient D of [BMIM]CI was calculated according to the model. The diffusion coefficient D decreased a little when the concentration of solution increased. As increasing the coagulation bath concentration, the diffusion coefficient D initially increased and then decreased, reaching a maximum of 5 wt% in the coagulation bath. The diffusion coefficient D decreased with the coagulation bath temperature. From the diffusion coefficient and the structure of the coagulated filament, it can be concluded that the diffusion rate of [BMIM]CI from PAN concentrate solutions is relatively slow, which is prospective to prepare uniform-structure fibers.