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For single‐atom (SA)‐based catalysis, it is urgent to understand the nature and dynamic evolution of SA active sites during the reactions. In this work, an example of Pt SA‐Zn0.5Cd0.5S (Pt SA‐ZCS) is found to display interesting phenomena when facing the Brownian collision of ions in aqueous photocatalysis. Via synchrotron radiation, surface techniques, microscopy, and theory calculations, the results show that two kinds of Pt sites exist: PtZn‐sub‐S3 (Pt substituting the Zn site) and Ptads‐S2 (Pt adsorbing on the surface). In Na2S, the S2− can coordinate with Pt atoms and peel them from the Ptads‐S2 sites, but leaves more stable PtZn‐sub‐S3 sites, bringing a low but stable catalytic activity (19.40 mmol g−1 h−1). Meanwhile, in ascorbic acid, the ascorbic acid ions show less complex ability with Pt atoms, but can decrease the migration barrier of Ptads‐S2 sites (67.18 down to 35.96 mmol g−1 h−1, 52.03% drop after 6 h). Therefore, the Ptads‐S2 sites gradually aggregate into nanoclusters, bringing a high but decayed catalytic activity. Moreover, a Pt SA‐ZCS‐Sulfur composite is designed mainly covered by PtZn‐sub‐S3 sites accordingly (max: 79.09 mmol g−1 h−1, 5% drop after 6 h and QE: 14.0% at 420 nm), showing a beneficial strategy “from mechanism to design principle.”

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