In this work, we present a novel facile strategy for green synthesis of polyethyleneimine (PEI)‐capped carbon dots (PEI‐CDs), in which citric acid and PEI were chosen as reactants and highly fluorescent PEI‐CDs could be readily obtained via a simple one‐pot refluxing under 120 °C within 2 H. Fluorescence studies indicate that the as‐prepared PEI‐CDs exhibit strong fluorescence emission at 446 nm with excitation at 365 nm. Upon the sequential addition of Cu2+ and H2S, PEI‐CDs result in an interesting “ON−OFF−ON” three‐state emission responses, promising a bifunctional sensory platform. Moreover, the Cu2+/H2S‐facilated reversible fluorescence changes of PEI‐CDs have demonstrated the design of an INHIBIT logic system based on Boolean logic.