Dye wastewater with high concentration, large molecular weight, complex structure, and certain toxicity has brought a serious burden to the environment. In this study, acid red 73 dye (AR73) was used as the model pollutant to compare the degradation effects of different systems on AR73. The influence factors of a UV/hydrogen peroxide/peroxydisulfate (UV/H₂O₂/PDS) composite system, such as H₂O₂ dosage, initial concentration of AR73, initial pH value, anion concentration, and actual water, on the degradation of AR73 were investigated. Compared with other systems, the UV/H₂O₂/PDS composite system had the best degradation effect of AR73, with a degradation efficiency of 98.98%, which conforms to the pseudo-first-order reaction kinetic model, the pseudo-first-order reaction kinetic constant (k_obs) is 0.1339 min⁻¹. And the synergy coefficients in this complex system were all greater than 2. Specifically, the degradation efficiency of the UV/H₂O₂/PDS composite system for AR73 was the highest when the initial pH was neutral. The presence of anions inhibited the degradation of AR73, and the real water reduced the degradation efficiency for AR73. Electron spin resonance (ESR) detection and free radicals capture test demonstrated that hydroxyl radical (^·OH) was the main reactive species in AR73 degradation, and its 80.66–84.93% contribution. With the destruction of the structure of AR73 by free radicals, anilines, phenols, and oxalic acid were found to be the main intermediate products during the degradation of AR73 by gas-phase mass spectrometry and UV–vis spectrograms. The changes in toxicity during the degradation of AR73 by the UV/H₂O₂/PDS complex system were also assessed.