Abstract: Based on systematic studies, sodium persulfate has a photochemical reaction to oxidize humic acid under alkaline conditions. The photochemical dynamic curve of Na₂S₂O₈-humic acid system has been established. The reaction conditions were optimized for negative high voltage, gain, pump speed, Na₂S₂O₈ and NaOH concentration. A new photochemical analysis method to determine humic acid in loess has been established with a linear range from 0.1 mg/L to 500 mg/L (R=0.9985) and a detection limit of 0.076 mg/L. Eleven replicate determinations were conducted on 0.5 mg/L humic acid, and the calculated relative standard deviation (RSD) was 3.47%. According to the determinations of humic acid from 5 different groundwater samples with an internal calibration, the standard recovery rates of the samples were between 98.33% and 107.50%. This method was successful in the determination of humic acid without any separation.