An instantaneous spectrum detection apparatus, consisting of monochromator, piezoelectric pressure sensor and oscilloscope was applied to study quick reaction and ignition delay time of benzene. On account of the pyrolysis of benzene easily driven under shock compression, a new method was introduced to determine ignition delay time of energetic materials and prevent advance trigger under incident shock wave. Comparison of calculated concentration of major species (C2) by the chemical kinetic program plotted against time with the experimental data recorded by oscilloscope. A fairly good agreement between calculated results and recorded data was obtained. It proved that using this apparatus can study the behavior of instantaneous emission spectrum preferably. Combination of experiment and chemical kinetic analyzed the reaction mechanism of formation of carbon in thermal decomposition process under shock wave.