Based on the kinetics of the treatment process of the completely mixed reactor in series, this study reveals the relationship between the reactor stages and the treatment efficiency, and it was applied to the simultaneous nitrogen and phosphorus removal process. The strengthening effect of the reactor stages of the main anoxic sections on the anoxic phosphorus absorption efficiency and the contribution to improving the treatment effect were investigated. Using sewage with a low carbon-to-nitrogen ratio as the research object and keeping the operation parameters of the improved anaerobic–anoxic–oxic (A²O) process unchanged, the experimental research was carried out under the condition that reactor stages in series of the main anoxic section were one, two, three and four, respectively. The results showed an increase in the number of reactors in series in the main anoxic zone. The total phosphorus (TP) concentration in the effluent of the main anoxic stage decreased significantly, and the phosphorus uptake increased from 4.411 g/d (when n; the number of reactor stages in series was one) to 5.086 g/d when n was 4. Additionally, the nitrate nitrogen (NO₃⁻–N) concentration in the effluent decreased, from 12.53 mg/L when n was one, to 9.62 mg/L when n was four, the removal rate of total nitrogen (TN) increased, from 56.86% when n was one to 65.98% when n was four, and the reduction power of nitrate nitrogen increased, and the denitrification rate increased. The increase in the number of reactors in series enhanced the anoxic phosphorus absorption and denitrification performance. Therefore, the main anoxic section of the synchronous nitrogen and phosphorus removal system can be designed and operated as reactors in series.