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Space‐time modulated metasurfaces enable efficient manipulations of nonlinear harmonics with more degrees of freedom than conventional materials by simply controlling the element geometries and modulation signals. The theoretical analyses reveal that a group of harmonics in reflected waves could be generated by the metasurface under the incidence of monochromatic wave with the rapid change of surface reflectivity, while the fundamental spectrum is greatly suppressed. However, it remains a great challenge to synthesize a high‐quality single‐tone signal for the reflected waves with excellent rejection ratio for higher‐order harmonics, which is highly desired for applications like wireless communications and radar detection. Here, a new scheme to overcome this limit, which experimentally realizes high‐efficiency frequency conversion from the fundamental harmonic to the order harmonic of …