Abstract: The mass transfer mechanism of extracting natural products with supercritical fluid was analyzed and the analytical solution of the dynamic model of supercritical fluid extraction was deduced through rationally simplifying the differential mass balance equations.A new dimensionless number Bz was proposed to characterize the dynamic mass transfer conditions of the process accurately, and a characteristic time t_k was found that -dr_c/dt reaches the minimum value when t=t_k, which might be the competing results of intraparticle diffusion resistance with intraparticle mass transfer driving force.The complete extraction time t_ex, cumulative extraction yield Y as a function of time t and the influence of flow rate Q on extraction yield Y were calculated with the model.The results of calculation fitted well with the experimental data of literature.This simple and reliable model could be used to predict ,analyze, design ,simulate and optimize the process of extracting natural products with supercritical fluid.