Additional operating parameters were determined as follows: Fe2+ = 0.3 mmol/L; PH = 3; UV = 40 W, and gas flow rate = 3 L/min. The solution volume = 6 L. Hydrogen peroxide varies from 0 to 30 mmol/L. H2O2 is a source of (•OH), so H2O2 concentration affects (•OH) generation [11]. The variation of the H2O2 concentration with time is shown in Figure 9, where the concentration of H2O2 in the solution decreases rapidly for the first 70 min when the H2O2 concentration is 10 mmol/L, after which the H2O2 is almost entirely consumed(close to 0 mmol/L at 100th min). When the H2O2 concentration increased to 30 mmol/L, the time that the liquid phase contained H2O2 increased to 130 min. The effect of the H2O2 concentration on the styrene effect of the solution is illustrated in Figure 10. With H2O2 at concentration 0, the outgassing styrene content increases rapidly to near the front-end level. As the concentration of H2O2 increases, we find that it has a minor effect on the optimal removal efficiency but prolongs the removal time for styrene. When H2O2 was 10 mmol/L, the styrene removed by the solution remained above 90% for the first 75 min from the beginning of the reaction. The removal efficiency decreased rapidly thereafter, dropping to 14% at 160 min. When the H2O2 concentration was 20 mmol/L, the average styrene removal efficiency reached up to 96.2% within 105 min after the reaction started, and the outlet gas styrene concentration began to increase significantly at about 105 min and finally decreased below 50% at about 140th min. As the H2O2 concentration is increased to 30 mmol/L, its average removal efficiency for styrene is as high as 95.7% within 130 min. Figure 9 and Figure 10 reflect the effect of H2O2 in the UV/Fenton system, with the degradation effect rapidly decreasing as H2O2 is fully depleted. The variation in the concentration of the by-products in the solution is shown in Figure 11. The TOC content in the solution was the highest at the H2O2 concentration of 30 mmol/L and as high as 88.79 mg/L at 130 min. When the concentration of the H2O2 was 10 mmol/L, the first 60 min TOC content was less than that of the solution with the H2O2 concentration of 20 mmol/L, but thereafter the TOC content in the solution with the H2O2 concentration of 20 mmol/L grew slowly. The final H2O2 concentration of 20 mmol/L solution showed a TOC of 22.45 mg/L at 140th min, less than 34.49 mg/L of 10 mmol/L. The efficiency of removal of styrene gas by different concentrations of H2O2 in this study is less varied. The effective removal time of the solution for styrene (removal efficiency greater than 90%) was not exponentially prolonged as the H2O2 exponentially increased. An H2O2 concentration of 20 mmol/L was chosen as the optimal concentration for photo-Fenton degradation of styrene gas by H2O2 in a nitrogen atmosphere, taking into account industrial economic benefits and by-product production.