When sulfuric acid is used to hydrolyze biomass, besides the macro factors (reaction temperature, acid concentration, reaction time, and ratio of solid to liquid), the diffusion rate of the acid into the biomass particle is also an important factor that affects the hydrolysis. In this article, pine wood sawdust was taken as test material (temperature: 433.15-463.15 K; sulfuric acid concentration: 10 Kg/m(3); ratio of L/S: 9; reaction time: 0-1,800 sec) and a distribution model of sulfuric acid was established by studying the diffusion characteristics in the heterogeneous phase consisting of sawdust and acid solution. Comparison between the calculated results from the model and experimental ones proved that the established model could satisfactorily predict the diffusion process of sulfuric acid into the sawdust particle during the reaction. Based on the distribution model, the effects of particle microstructure on the sulfuric acid concentration distribution were investigated. It indicates that particle size and porosity are the important factors that affect sulfuric acid diffusion in hydrolysis and the effect of porosity includes that from tortuosity. This study has a numerical description of sulfuric acid distribution in the bulk flow and within the particle during the reaction, and is of great significance in the further study of biomass hydrolysis mechanism.