1,3,5-Triamino-2,4,6-trinitrobenzene (TATB) is a typical insensitive high explosive (IHE) that possesses excellent heat, impact, and shock stability. However, it is sensitive to light irradiation, which can produce a long-lived free radical. In this study, (time-dependent) density functional theory is employed to study the features of the ground state (S0), the first singlet excited state (S1), and the first triplet excited state (T1). Results indicate the progress of photolysis, which involves an intersystem crossing from S1 to T1 followed by NO2-ONO isomerization. The long-lived radical produced in TATB photolysis is identified as the phenoxyl radical R-NO by investigating the formation feasibility and the stability. The experimental observation of the changes in the ultraviolet absorption spectra of TATB supports the identification.