<p>Blue fluorescent carbon dots (Soy-His CDs) with high fluorescence quantum yield were synthesized via a hydrothermal method using biowaste soybean straw and L-histidine as precursors. A ratiometric fluorescence probe (Soy-His CDs@PEp-PEI) was subsequently constructed for the visual detection of epinephrine (Ep). Upon the continuous formation of the PEp-PEI copolymer, the green fluorescence of the Soy-His CDs@PEp-PEI probe at 515&#xa0;nm gradually intensified, while the blue fluorescence at 420&#xa0;nm was progressively quenched. This allows for the rapid and sensitive detection of Ep by monitoring the variation in the I₅₁₅/I₄₂₀ ratio. The limit of detection (LOD) was determined to be 0.025 µmol/L in the linear range 0.1–20 µmol/L. The response mechanism is attributed to the inner filter effect (IFE) between the Soy-His CDs and the polyepinephrine-polyethyleneimine (PEp-PEI) copolymer. The proposed method was successfully applied to detect Ep in human urine samples, yielding recoveries ranging from 95.82% to 102.01%. Furthermore, portable visual and intelligent detection of Ep was achieved using a smartphone equipped with RGB analysis software based on the Soy-His CDs@PEp-PEI probe system, with recoveries ranging from 94.65% to 108.33%. A good linear relationship was obtained in the range 0–10 µmol/L, with a limit of detection (LOD) of 0.14 µmol/L.</p> Graphical Abstract <p></p>