Tailoring the pore structure is a straightforward and effective strategy to regulate the catalytic activity of catalysts in heterogeneous catalysis reactions but remain a challenge. Herein, we present a low-cost, facile, and large-scale mechanochemical nanocasting strategy between cerium salt and manganese salt that enables solvent-free and rapid synthesis of porous CeMnOx oxide with a high specific surface area, rich chemisorbed oxygen species, Mn4+, and acidity, as well as improved reducibility. Moreover, this kind of catalyst shows exceptionally high low-temperature activity for the selective catalytic reduction of NOx with ammonia (NH3-SCR). It achieves almost 100% NOx conversion at 150 degrees C, which is much lower than that of its bulk CeMnOx counterpart constructed by the sol-gel method. It is expected that this technology may open up new opportunities for fabricating a number of advanced porous materials with exceptional catalytic activity for the environmental catalysis field.