At present, ionic liquids’ high production and use costs are limiting their large-scale applications, so how to recycle them has attracted great attention. Crystallization processes are extremely important for developing new recycling technologies for ionic liquids, and cooling rates have important effects on the crystallization processes. Based on these, in this paper, polarizing microscopy, small angle X-ray scattering and Raman spectroscopy were employed to research the phase transitions and the structural changes of 1-dodecyl-3-methylimidazolium tetrafluoroborate (［C12mim］［BF4］) from 60 to 0 ℃ at the cooling rates of 30 and 1 ℃·min-1, in order to reveal the effect of cooling rates on the crystallization process and product. POM results show that ［C12mim］［BF4］ experienced the phase transitions from the liquid state to liquid crystal state and then to crystal state Ⅰ during the rapid cooling process, and that ［C12mim］［BF4］ underwent the phase transitions from the liquid state to liquid crystal state and then to crystal state Ⅱ during the slow cooling process. The crystal state Ⅰ consisted of many “ball-like” crystals with large sizes, while the crystal state Ⅱ was composed of a lot of “needle-like” crystals with small sizes. In addition, SAXS results show that ［C12mim］［BF4］ has two crystal structures, including perpendicular and triclinic bilayer phases. Both were found simultaneously in the crystal state Ⅰ, but only the triclinic bilayer phase appeared in the crystal state Ⅱ. Therefore, crystal state Ⅰ is a mixed phase crystal, while crystal state Ⅱ is a single phase crystal. Furthermore, it can be concluded from the Raman results of ［C12mim］［BF4］ that the ［C12mim］+ in the perpendicular bilayer phase is the G conformation, and that the ［C12mim］+ in the triclinic bilayer phase is the A conformation. In conclusion, ［C12mim］［BF4］ underwent the phase transitions from the liquid state to liquid crystal state and then to crystal state Ⅰ at a rapid cooling process, and a mixed phase crystal composed of the perpendicular bilayer phase and the triclinic bilayer phase was obtained. However, ［C12mim］［BF4］ underwent the phase transitions from the liquid state to liquid crystal state and then to crystal state Ⅱ at a slow cooling, and a single phase crystal consisting of the triclinic bilayer phase was obtained. What is more, the mixed phase crystal includes the G conformation and the A conformation of ［C12mim］+, while the single phase crystal only contains the A conformation. So, the cooling rate has an important effect on the crystallization process and product of ［C12mim］［BF4］. These results provide important experimental data for enhancing the recovery technology of ［C12mim］［BF4］, and are also helpful in investigating the phase transition and structure change of similar ionic liquids.