UV-based advanced oxidation processes are promising in micropollutant removal from water with abundant relative researches accomplished in batch reactors, while the different flow pattern in practical flow-through UV reactors may lead to a varied reaction kinetic and process efficiency. In this study, the kinetics of atrazine (ATZ) degradation in flow-through UV/H₂O₂ reactors were investigated, and the impacts of H₂O₂ concentration and internal reactor diameter on ATZ removal efficiency and the process energy efficiency were evaluated. The steady-state assumption (SSA) model was developed to predict the degradation kinetics of atrazine under various experimental conditions, and its accuracy was tested by comparing with experimental data. The results showed that the efficient degradation of ATZ occurred in flow-through UV/H₂O₂ reactors and it followed the pseudo first-order kinetics (R²>0.95). Despite the fact that the flow was not fully mixed, SSA model exhibited good accuracy in predicting the degradation of target pollutant in flow-through reactors with deviations generally less than 20%. Within the investigated concentration range, ATZ degradation rate in different reactors increased with the rising of H₂O₂ concentration, and reached 5.8×10⁻² s⁻¹ in the reactor with an internal diameter of 35 mm when H₂O₂ concentration was 0.2 mmol·L⁻¹, which was 4 times as much as that in UV radiation alone. The increasing reactor diameter resulted in a low time-based ATZ degradation rate constant on account of the changes in average fluence rate, while had slight effect on the fluence-based ATZ degradation rate constant. Finally, based on the electrical energy per order (E_EO) calculation, the energy efficiency in removing ATZ can be reduced by increasing H₂O₂ concentration and reactor diameter.