Abstract: The tortuosity factor is an important parameter of the catalyst and it has an effect on the global reaction rate of gassolid catalytic reaction, the capacity of the reactor and the efficiency of the catalyst etc.. The tortuosity factor of A301 ammonia synthesis catalyst is measured using dynamic state methodsingle pellet string reactor method (SPSRM). More than 50 particles of A301 catalyst reduced are packed one by one in a chromatography column. The trace gas is injected into the column. The adsorption, desorption and diffusion occur in the catalyst. The pulseanswer curves are autoanalyzed by online computer system so that the first absolute and the second central moments are attainable. The parameter estimation method is applied to get the effective diffusion coefficient from the experimental data, so that the tortuosity factor can be obtained. The tortuosity factor of A301 catalyst is 3.28.