The structure of 3,4-bis (3′-nitrophenyl-1′-yl) furoxan was estimated by a B3LYP method based on 6-31G(d,p) basis set and the stable geometric configuration and Wiberg bond order were obtained. Thermal decomposition mechanism of 3,4-bis (3′-nitrophenyl-1′-yl) furoxan was studied by means of simultaneous TG-DSC-FITR-MS. In the geometric configuration after optimization,the atoms of furoxan are in one plane and N—O coordination bond is similar to double bond because of conjugate. The bond order of the N—O single bond near N—O coordination bond is 0.8769. The initiation step of thermal decomposition is the cleavage of incited N—O single bond. Thermal decomposition mechanism of the title compound was supposed,namely at first,furoxan ring fission occured and produced some small molecular gas and nitrobenzol,then nitrobenzol decomposed by two routes that either free radical cleavage or isomerization to nitrosophenol.