To study the influence of crystal defects on the initial reaction of hexanitrohexaazaisowurtzitane （CL-20）， molecular dynamic simulation and ReaxFF-lg reactive force field are used to study the initial reaction path， thermal decomposition products， and reaction kinetics of CL-20 with vacancy defects at high temperature （1500-3500 K）. The results show that the initial decomposition path of CL-20 with vacancy is the breaking of N─NO₂ bond， the same as that of perfect crystal. The vacancy defects prove to increase the frequency of ring-opening reactions and the production of NO₂. Compared with perfect CL-20， it can be seen that the vacancy defects would reduce the CL-20 activation energy barrier and accelerate its thermal decomposition process. The reaction rate constants of CL-20 with 16.7% vacancies are 1.7 and 1.4 times higher than that of perfect CL-20 at 2000 K and 3000 K， respectively. The CL-20 molecules around the vacancy are easier to decompose， leading to the increase of the sensitivity of CL-20.