Abstract: To decocted the block (2 mm<d<5 mm) of Rheum of ficinale, after that to take a sample at different decocting time. Methods A 722E spectrophotometer is analyse the anthra quinones was used to determine at different decocting time, while the time-content relation was analyzed. Results is the Rheum of ficinale decocted for 30min before, the content of free anthrax quinones increased rapidly reached the peak value at the end of 50min. Conclusion Rheum of ficinale and the dynamic changes of the decocting time. According to the experimental results, 50min is considered to the decocting time when Rheum of ficinale is used cathartic.