The combination of UV irradiation and chlorine (UV/Cl₂), as a new and efficient process, has been widely applied in drinking water disinfection. In this study, an improved VRC-3R⁻ (variable rate coefficient – 3 reactants minus other reactants) residual chlorine decay model was proposed in the UV/Cl₂ process in a water distribution network. Firstly, the single-factor experimental method was employed to assess the effects of initial residual chlorine concentration in the UV disinfection stage (C₀₁) and pipe network decay stage (C₀₂), UV intensity (F), inorganic nitrogen ([NO₂⁻]), ammonia nitrogen ([NH₄⁺]), and total organic carbon (TOC) on residual chlorine decay during the UV disinfection stage and pipe network decay stage. Next, the response surface method (RSM) was used to establish multivariate functions among the decay coefficients in the UV disinfection stage (k) and pipe network decay stage (K), C₀₁, C₀₂, temperature (T), and pH as correction coefficients. Finally, the model was calibrated using EPANET 2.0. In conclusion, (1) the optimal residual chlorine concentration and UV dose were 7.75 mg L⁻¹ and 50.1 mJ cm⁻², respectively; the pipe network disinfection by-product safety was confirmed. (2) The longest hydraulic retention time (HRT) in the pipe network was 5.12 h before the secondary chlorination. (3) The additional decay model by UV irradiation was established by subtracting the residual chlorine decay function with and without UV irradiation using the RSM's fitting results. (4) The calibration function value of the new model for two cases reached δ_1win (C) = 0.18, δ_1sum (C) = 0.16, δ_2win (C) = 0.16 and δ_2sum (C) = 0.14 in summer & autumn and spring & winter, respectively, which were less than the minimum (0.33) and standard values (0.25). (5) The residual chlorine concentration at the end of the water distribution network was less than 0.1 mg L⁻¹ in summer & autumn, exhibiting the importance of secondary chlorination to provide a reference for this process's application in water distribution networks.