The annual treatment capacity of the four sewage treatment plants in Lanzhou city accounts for more than 95% of the total sewage treatment capacity of the urban sewage treatment facilities in Lanzhou city. The study of the effluent discharge limits of the four sewage treatment plants is of great significance to the improvement of water environmental quality and the implementation of accurate and scientific pollution control in the Lanzhou section of the Yellow River. The research shows that the effluent water quality should not be blindly pursued and the standards should be upgraded blindly. The carbon emission during the operation of the sewage treatment plant should be comprehensively considered, and the carbon emission should be taken into account under the premise of meeting the water quality objectives of the receiving water bodies. Based on the above understanding, the main pollutant emission concentration of the external drainage of four sewage treatment plants in Lanzhou City during 2019-2021 was statistically analyzed, and the effects of water reuse and emission reduction in the four sewage treatment plants and the impact of discharge into the river were analyzed. The water reuse rate of four sewage treatment plants in Lanzhou city is low, ranging from 0.86% to 4.66%. The average annual COD and ammonia nitrogen emission reductions of the four sewage treatment plants were 114584.1t and 8234.6t, respectively, showing obvious emission reduction effects. The four sewage treatment plants are discharged according to the design scale and design effluent quality. Under the superimposed influence of other inlet and outlet in the range, the discharge will affect the water quality of the downstream reaches during the low temperature season of the Yellow River and under the condition that the upstream incoming water quality approaches or exceeds the target limit value of Class II water quality. Based on the above studies and the research results on carbon emissions in the operation stage of sewage treatment plants, the effluent emission limits of four sewage treatment plants in Lanzhou city are proposed based on the target constraints of carbon pollution reduction and water quality. It is suggested that effective measures should be taken to promote the synergic effect of carbon pollution reduction and carbon reduction during the operation of sewage treatment plants, and constructed wetlands should be constructed according to local conditions to improve the water quality of external drainage. Actively expand the ways of reclaimed water reuse, increase the reuse rate, and reduce pollutant discharge.