Abstract: In the medium of 5.5×10^-2 mol/L HCl, there is a max absorption peak at 370 nm for the system of 2.8×10^-3 mol/L KI-2.4×10^-4 mol/L HAuCl₄. A spectrophotometric method was proposed for the determination of alkaloids in Beidougen(BA) capsule, based on the formation of BA-AuI₂^- association particles which have a absorption peak at 538nm. The linear range is 0.3~18 mg/L(γ=0.9992,n=6), with a mole absorptivity of 3.57×10⁴ L/(mol/cm). This method has convenience, rapidity, accuracy and good recovery. It was applied to the determination of total alkaloids in Beidougen Capsule with satisfactory results.