The varied-valence nanocatalyst MnO_x was prepared using the oxidation-reduction method under ultrasonic. The optimal synthetic conditions of MnO_x were explored by adjusting the ultrasonic time, concentration of reaction precursor, drying temperature, and pH of reaction solution. The results indicated that the optimal sample MnO_x was synthesized under the condition of ultrasonic time of 20 min, 0.5 mol/L KMnO₄, drying temperature of 80 ℃ and pH=7, which showed the super catalytic performance and the time of 100% NO removal rate was as high as 15 h at room temperature. The structure and morphology of the optimal catalyst MnO_x were investigated by XRD, N₂-adsorption-desorption analysis, SEM and TEM. Besides, XPS and FT-IR were also applied to explore the catalytic oxidation process of NO removal and the deactivation mechanism of the optimal sample MnO_x. It is believed that the interpenetrating and hierarchal pore, the petaloid morphology and weak crystallization structure contribute to the gas adsorption and transmission. The presence of varied-valence Mn and oxygen vacancy can improve the adsorption and activation of NO and O₂, thus, enhancing the NO catalytic removal on the optimal catalyst MnO_x at room temperature.