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 (S-0), the first singlet excited state (S-1), and the first triplet excited state (T-1). Results indicate the progress of photolysis, which involves an intersystem crossing from S-1 to T-1 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.