The thermal behavior of guanidinium azotetrazolate (GZT) was studied using the thermogravimetry and differential scanning calorimetry. The thermal decomposition mechanism of GZT was proposed based on the investigation of products in condensed and gas phases by the combination techniques of solid in situ cell with the rapid-scan Fourier transform infrared spectroscopy and pyrolysis-gas-chromatography-mass-spectrometry (Py-GC-MS). Results show the first mass loss process of GTZ includes the exothermic decomposition reaction of azotetrazolate and the endothermic decomposition reaction of guanidinium. Under heating, the intermediate product tetrazoleazide (CHN₇) may forme through the ring-opening reaction of tetrazole on the azotetrazolate molecular and the ultimate products are high boiling point materials: melamine and ammonium azide. Under the bombardment of ion source, the bond C—N_azo on azotetrazolate molecular also split and dissociate in sequence.