Compared with O2-dependent chemiluminescence (CL) and bioluminescence (BL), H2O2-dependent ones are far from being completely understood. A two-step mechanism for H2O2-dependent CL production from tetrachloro-1,4-benzoquinone (TCBQ) was proposed based on experimental evidence of detecting the formation of several intermediates and products. This mechanism is not yet supported by theoretical evidence, and its details remain unknown. In the present paper, we performed multireference and (time-dependent) density functional theory calculations on the complete reaction process of TCBQ with H2O2 to produce CL. The calculations reproduced the experimentally observed two-step CL. Although the reactants are different, the first and second CLs follow very similar reaction processes and mechanisms. First, an anionic dioxetane is formed via five sequential reactions. The intrinsically produced •OH is crucial for forming dioxetane. Subsequently, the anionic dioxetane decomposes to produce an anionic excited-state (S1) product. A conical interaction of the ground and the S1-state potential energy surfaces is responsible for producing the S1-state product. Finally, the S1-state anionic product changes to its neutral form, and the latter emits light as an actual light emitter. This mechanism could be extended to luminescent systems of all H2O2-dependent tetrahalogenated quinoids, including acorn worms, because TCBQ/H2O2 is a typical representative of these luminescent systems.