The thermal decomposition process of hexanitrohexaazaisowurtzitane (HNIW,CL-20) under vacuum was investigated by a new gasometric instrument（LAWA） for testing material thermal stability. The experiment shows the V_max is 628.5 mL·g^-1 and the decomposition reaction for the 0-50% range of conversion degree of CL-20 follows the Avrami-Erofeyev equation in the temperature range 170-200 ℃; the activation energy E_a is 174.1 kJ·mol^-1 and ln(A/s^-1) is 42.34. The results was compared with that by Bourdon manometer,isothermal thermogravimetry (TG) and DSC methods.