Productive commissioning of treating sewage in plateau region has the features of low temperature, low atmosphere pressure and low oxygen concentration. The CASS process was applied in the expansion of wastewater treatment plant in ShangriLa. Aiming at the low air temperature and low atmosphere pressure and the low concentration of influent, several approaches, such as adjusting the aeration mode compatibly, controlling the contents of dissolved oxygen and cultivating active sludge with warm maintenance, were studies for system stable operation under the condition of mismatch between design parameters and operation mode. With the field test, when the water quality of influent was: CODCr:50.0~164.0 mg/L, SS:30.0~123.0 mg/L, TN:7.0~82.6 mg/L, NH3N:1.9~39.2 mg/L, TP:0.6~3.2 mg/L, the quality of effluent was: CODCr:10.0～49.0 mg/L, SS:5.0～22.0 mg/L,TN:3.4～22.7 mg/L, NH3N:0.9～11.1 mg/L, TP:0.1～1.8 mg/L, in which the standard of the level B in class I were achieved according to Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002).