Objective To study the change of benzoic acid content in water extract of Paeoniae Radix Alba after decocting, and the influence of different decocting time on the content of benzoic acid. Methods The raw materials were decocted separately or in combination, and concentrate to a density of 1.15-1.20, the content of benzoic acid in the water extract after decocting and concentrating was determined by high performance liquid chromatography. Results Benzoic acid content was not detected (limit of quantification was 0.01 g/kg) in Paeoniae Radix Alba, the benzoic acid content in concentrated liquid of paeony increased to 2.91-3.77 g/kg after single decocting, and the benzoic acid content after two-stage decocting ranged from 3.26 g/kg to 4.02 g/kg. The content of benzoic acid in the mixed concentrate containing Paeoniae Radix Alba was 0.542 g/kg. Benzoic acid was not detected in others. Conclusion The content of benzoic acid in water extract of Paeoniae Radix Alba increased after high temperature decocting, and it's proportional to time. Therefore, when preparing food or health products with Paeoniae Radix Alba as the main raw material, attention should be paid to the control of decocting temperature and time, and the factory inspection of this item should be increased to prevent the harm caused by high content of benzoic acid.