Abstract: Water environment capacity (WEC) is an important theoretical basis of total pollutant control, and the selection of the design hydrological condition is the key step to calculate the WEC. A steady state model of water quality in Tieling section of the Liaohe River Basin was built, and the WEC of Tieling control unit was calculated under 30B3, 4B3, 7Q10, 30Q10 design hydrological conditions. Under the 30B3 and 30Q10 design hydrological conditions, the WEC of COD was 8 048.74 and 9 658.49 t/a, respectively, while that of ammonia nitrogen was 549.15 and 658.97 t/a, respectively. Under the 4B3 and 7Q10 design hydrological conditions, the WEC of ammonia nitrogen was 439.33 and 494.26 t/a, respectively. The WEC calculated by traditional hydrological method is similar with the result of ecological security method, but the daily flow guarantee rate of traditional hydrological method is relative low, which leads to the increase of pollution prevention and control risk. According to the calculation result of WEC and pollution load discharge into the river, the development of Tieling City has exceeded the water environment carrying capacity. In order to increase the pollution carry capacity and achieve the pollution prevention objectives, it is imperative to optimize the adjustment capacity of the reservoirs and rationally allocate the water resources.