pH dependent fluorescence emission of a thioxanthone-based probe has been reported recently. To reveal potential determinant factors of this pH dependence can provide important clues to design novel thioxanthone-based probe in the future. Thus, pH dependence on photochemical kinetics of thioxanthone itself was investigated in this work, using nanosecond time-resolved laser flash photolysis. The nanosecond time-resolved transient absorption spectra and kinetics of TX in aqueous acetonitrile were recorded, as well as for a model reaction system including TX and diphenylamine (DPA) as co-initiators. Besides the well-known absorption peaks of <sup>3</sup>TX<sup>*</sup>, the other peaks at 417, 518, 673 and 780 nm, have been reliably attributed to major intermediates in the overall reaction between TX and DPA with photolysis, which has been confirmed to occur along a multistep process. In the strong acidic solution (pH ~3.0), TX and TXH<sup>+</sup> coexist due to protonated equilibrium. Consequently, high proton concentration promotes the predominant decay pathway after photolysis from electron transfer to proton affinity. Subsequently, the different primary products, <sup>3</sup>TXH<sup>+*</sup> or TX<sup>•−</sup>, proceed different secondary reaction channels. In addition, within the wide pH range from weak acid (pH 5.0) to alkaline solution (pH 13.0), the overall reaction mechanism and rates do not show visible changes.