Objective: To provide scientific foundation for establishing harvesting time of best quality of herb. Method: Protocatechuic acid was determined by HPLC to study dynamic trends of its content of the Cynomorium songaricum in different growth stages. Using Eclipse XDB C18(4.6 mm×250 mm

5 μm)as the stationary phase

acetonitrile-water-glacial acetic acid(10 ∶100 ∶0.1)as the mobile phase at a flow rate of 1.0 ml ·min-1

259 nm as the detection wavelength

the injection volume was 10 μL. Result: The protocatechuic acid content of C. songaricum Rupr. changed regularly. The content was higher at preemergence stage than that of unearthed. The content at unearthed stage was higher than that in flowering stage. And In flowering stage was higher than fruiting stage. Conclusion: Protocatechuic acid could be regarded as one of mark components

that the preemergence stage was the best. The research provided a scientific foundation for the right harvesting -time and the quality evaluation of Herba cynomorii.