Abstract: Artificial calculus bovis (ACB) is widely used in dispensing Chinese Traditional Patent Medicine as a substitute for Calculus bovis. Photodecomposition rule of ACB sample irradiated with three different light sources was studied by diffuse reflectance spectrophotometry (DRS) in this paper. The results show that, the photodecomposition rate curves of the ACB sample irradiated by all three light sources are composed of two straight lines of different slopes, indicating that they are of two-step apparent first order reaction, the apparent photodecomposition constants in the first steps are about twice as high as those in the second steps. The ACB sample has the fastest photodecomposition rate when irradiated with the UV mercury-arc lamp and the second with the fluorescent mercury-arc lamp and slowest with the iodine-tungsten lamp. In these three light sources apparent photodecomposition constants of ACB sample are respectively: K₁, 2.7629×10^-5, 4.4132×10~(-6); k₂, 1.2176×10^-5, 2.0684×10^-6, 1.4357×10^-6 (lx^-1·h^-1). The apparent constants are independent of the product of the radiation intensity and the irradiation time, but the irradiation time is in inverse ratio of the radiation intensity to get the same proportion of the sample photodecomposition. The fading time of the ACB sample under different radiation intendity can be predicted with the kinetic equations reported. The fading time of ACB sample if directly exposed to light in bright room is 1.6 days. ACB is a complex prescription, in which bilirubin is the main component bearing the photodecomposition, the others are almost colorless and do not bear the photodecomposition which changes the colour of the ACB sample. However, they obviously accelerate the photodecomposition of bilirubin in the ACB sample in the second step of the two-step apparent first order reaction.