The invention discloses a closed-loop control system for treating landfill leachate in cooperation with partial nitrosation and denitrification and a denitrifying and decarburizing method for treating the landfill leachate. A programmable logic controller in the system is connected with a water inlet pump, a water outlet valve, a mud valve, a stirrer, a fan, a water inlet/outlet and an alarm floating ball switch, a sensor automatically controls the process of a reactor such as water charging, stirring, aerating, depositing, mud discharging, and water discharging, and the system is provided with a constant temperature system to control the reaction temperature. The denitrifying and decarburizing method comprises the following steps of: firstly stirring in an anaerobic way to denitrify partial nitrite nitrogen and nitrate in the reactor after the water charging, and aerating to degrade the residual organic matter and carry out partial nitrosation, so that the most degradable organic matters can be removed, and about 53% of ammonia nitrogen can be converted into nitrite nitrogen, therefore, a condition can be created for the anaerobic ammonia oxidation treatment. The system can carry out the stable denitrification treatment and the partial nitrosation treatment on the landfill leachate, can further effectively remove the organic mattes, and is high in mud settleability, high in activity, full-automatic in control, convenient and reliable.