Potassium doped graphitic carbon nitride (g-C3N4) sludge-based composite photocatalyst adsorbent (AC/K-CN) was prepared by calcination method using flocculation sludge of water-based ink wastewater, melamine and potassium carbonate as raw materials. The crystal structure, morphology and photoelectric properties of the composite were characterized by X-ray diffraction (XRD), ultraviolet-visible diffuse reflectance (UV-vis DRS), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), specific surface area (BET), scanning electron microscope (SEM) and fluorescence (PL) spectroscopy. Meanwhile, the composite was applied to remove Cationic Blue X-GRRL from aqueous solutions by photocatalyst under visible-light irradiation. The results show that the photocatalytic activity of the AC/K-CN composite catalyst on cationic blue X-GRRL is significantly higher than that of pure g-C3N4 or AC, which is attributed to the effective electron-hole separation of the composite material. Compared with pure g-C3N4 and AC, the AC/K-CN composite catalyst increased the decolorization rate of cationic blue X-GRRL with a mass concentration of 20 mg/L by 43.17% and 46.76% under simulated sunlight.