Objective To establish a thin layer chromatography scanning method for the determination of cinnamaldehyde content in Qianzheng Plaster.Method Cinnamaldehyde was extracted from Qianzheng Plaster by distillation method.Silica G was used as the thin-layer plate;petroleum ether (60～90℃) and acetic ethyl acetate (17 ∶3)served as the mobile phase and the scanning wavelength was 269 nm.Results A good linearity was obtained in the range of 0.81?g～4.05 ?g (r=0.9993).The mean recovery rate was 95.23 %and RSD=1.39 %.Conclusion This method is accurate and sensitive with good reproducibility.