Thermal decomposition of hexanitrohexaazaisowurtzitane (HNIW) was investigated through tuneable vacuum ultraviolet photoionization with molecular-beam sampling mass spectrometry (MBMS). According to photoionization efficiency (PIE) spectroscopic results, the initial decomposition products of HNIW were identified including HCN, CO, NO, HNCO, N(2)O, CO(2) (a little), NO(2), C(2)H(2)N(2), C(3)H(3)N(3), C(4)H(3)N(3), C(3)H(4)N(4,) C(5)H(4)N(4), C(5)H(5)N(5) and C(6)H(6)N(6). The possible ionization energies of C(2)H(2)N(2), C(4)H(3)N(3), C(3)H(4)N(4) and C(6)H(6)N(6) were analyzed on basis of the PIE spectra. The data were compared with those of thermogravimetry-mass spectrometry (TG-MS) and thermogravimetry-Fourier transform-infrared spectroscopy (TG-FT-IR). The kinetic parameters for the formation of HNCO, HCN and CO(2) were calculated from the current curves of species by TG-FT-IR spectroscopy, typically the apparent activation energy (E(a)) and prefactor (A) for HNCO were E(a)=161.3 +/- 2.5 kJ mol(-1) and A=38.9 +/- 0.6 s(-1) with an optimal mechanism function f(alpha)=(1-alpha). Global thermal decomposition reaction and Arrhenius equation of HNIW were suggested at the end.