The functionalized graphene oxide-ZnO (fGO/ZnO) nanoflower composites have been studied as a photocatalyst material for flow photodegradation of humic acid (HA) in real samples. The fGO/ZnO nanoflower was prepared via hydrothermal methods. The chemical and physical properties of the synthesized photocatalyst have been carried out by several techniques, including X-ray diffraction (XRD), scanning electron microscope-energy-dispersive spectrometer (SEM-EDS), Fourier transform infrared (FTIR), and UV-Vis spectrophotometer. The photocatalytic study of degradation of HA by flow system is reported. The optimum condition for degradation was found at pH 4.0, a flow rate of 1 mL min⁻¹, and a light intensity of 400 mW cm⁻². The degradation efficiency of HA also was influenced by several anion or cation concentration ratios on the system. This method was applied for the degradation of HA in extracted natural HA from the soil, and the efficiency achieved at 98.5%. Therefore, this research provides a low-cost, fast, and reusability method for HA degradation in the environment.