Enzymatic hydrolysis of konjac glucomannan（KGM）by β-mannanase was studied at constant mass ratio of enzyme to substrate to investigate the reaction-diffusion mechanism.By using size exclusion chromatography（SEC），the changes in molecular weight distribution of KGM were followed during the process of hydrolysis.When the ratio of enzyme to substrate was constant，the changes in molecular weight at different KGM concentrations were remarkably different，which showed that the reaction depended on the diffusion of enzyme.The changes in molecular weight showed a zero-order kinetics.Integrating the empirical reaction kinetics and diffusion characteristics，a reaction-diffusion model was established from a series of chromatographic results.But the degradation rate was shown to be greatly affected by the diffusion of enzyme as well as the concentration of substrate.This equation of reaction kinetics indicated the importance of diffusion，and the experimental data can be well predicted at semi dilute concentrations of KGM.