Using maize stover as the biomass, the heat- and mass-transfer transients within the particle microstructure were studied with a spatial kinetic model, in order to improve the efficiency and accuracy of real-time techniques. Voids were introduced into an idealized sphere to establish the kinetic model. The results indicated that particle size and porosity are critical parameters for mass and heat diffusion during hydrolysis. This study provides numerical descriptors within the biomass particle, and it is significant for predicting the biomass hydrolysis process.