A device and method for shortcut nitrogen removal and nitrite-oxidizing bacteria activity inhibition are disclosed herein. An embodiment of the present invention provides a yarn fiber diffuser comprising: a plurality of yarn fibers on which bacteria can be attached and grow; and an inlet capable of supplying gas to one sides of the plurality of yarn fibers, wherein the gas includes oxygen and carbon dioxide, nitrite can be produced by the oxygen, and the concentration of oxygen in the gas is adjusted by the oxygen and the carbon dioxide.