The NOx storage features and mechanism of a K-doped [Ca24Al28O64]4+·4O– catalyst (designated as C12A7-O–/K) were systematically investigated. The NOx storage capability (NSC) of the C12A7-O–/K catalyst was found to depend primarily on the temperature, the K-doping level, and the oxygen concentration in the gas mixture. The C12A7-O–/10%K catalyst exhibited the highest storage capacity among the C12A7-O–/M (M = K, Na, Mg) catalysts investigated, attaining a maximum NSC value of 183.9 μmol/g at about 550 °C. The optimum temperature window for NOx storage with C12A7-O–/10%K catalyst ranged from 400 to 650 °C. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) spectroscopy were utilized to identify the catalyst characteristics and the active species, both in the body of the catalyst and on its surface. Additionally, anionic time-of-flight mass spectrometry (TOF-MS) measurements were employed to investigate the anionic intermediates desorbed from the catalyst surface. It is proposed that NOx is mainly adsorbed on the K2O sites; O– anions, emitted from another NOx storage site on C12A7-O–, promote the formation of the NOx storage intermediates NO2, NO2–, NO3–, and so on, thereby boosting NOx storage.