The crystal structure, thermal stability, and properties of all polymorphs (including α-, β-, γ-, ε-and ζ-) of CL-20 were introduced. Recrystallization method of CL-20 including solvent-non-solvent method and solvent evaporation method, and the polymorphic transformation of CL-20 in solution were summarized. Considering that ε-CL-20 has the most application value and cannot be prepared directly. It is still necessary to be recrystallized but the polymorph of CL-20 would transform to others in this process which could cause the polymorph purity of ε-CL-20 decreasing, and then affecting the explosive performance of CL-20. The factors affecting the polymorphic transformation of CL-20, such as solvent and temperature were discussed. According to the Ostwald′s rule, the polymorphic transformation of CL-20 in solvent could be explained from the aspects of kinetics and thermodynamics. The deficiency of current research was depicted and the preparation way of ε-CL-20 with high purity through controlling the polymorphic transformation path and optimizing the recrystallization conditions were proposed.