The molecular dynamics simulations with condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field have been employed to study high energetic material CL-20. The lattice parameters and elastic constants in the pressure range of 0~10 GPa (for ε-CL-20) and 0~20 GPa (for α-, β-, and -CL-20) at room temperature, and in the temperature range of 100~500 K at room pressure are predicted. The good agreement of lattice parameters, pressure-volume relation and elastic constants of the greatest stability phase ε-CL-20 shows the accuracy of our simulations. The theoretical predictions of lattice parameters and mechanical properties of CL-20 under temperature and pressure can provide powerful guidelines for the engineering application and await further experimental confirmation.