In order to analyze high temperature phase transformation of hexanitrohexaazaisowurtzitane （CL-20）， phase transformation temperatures and coefficients of thermal expansion of ε-， β-， and γ-CL-20 were studied via ReaxFF-lg reactive force field molecular dynamics， with modified valence potential intercept. To validate the applicability of selected force field，the density， cell constant， lattice energy， and sublimation enthalpy for three types of CL-20 at room temperature were calculated. The third order Birch-Murnaghan equation of state was used to fit the p-V curve of ε-CL-20， with pressure ranging from 0 to 280 GPa. And the variation of bulk modulus （B₀） and its partial derivative to pressure （B′₀） with the increase of pressure is analyzed. High temperature phase transformation analysis shows that ε-and γ-CL-20 change phases at 398-423 K， of which the ε→γ phase transition occurs at atmospheric pressure， while the γ→ε phase transition needs 0.5 GPa or higher pressure； β-CL-20 transforms to ε crystal form at 448 K. The thermal expansion coefficient analysis shows that there is no obvious anisotropy in the high temperature thermal expansion process of ε-CL-20， while β- and γ-CL-20 show anisotropy in c direction and b direction， respectively. Results show that the modified ReaxFF-lg reactive force field is suitable for the study of phase transition of ε-， β-， and γ-CL-20 at high temperature and high pressure， while the accuracy of thermal expansion of β- and γ-CL-20 needs to be further improved.