The bench-scale pyrolysis of lignocellulosic biomass was investigated based on effect of thickness by both the experiment and numerical simulation. A critical thickness at which the two peaks of mass loss rate start to merge and the pyrolysis process is significantly accelerated, is paid attention in the fire propagation apparatus experiment at N-2 atmosphere. A new method is put forward to predict the merge thickness by coupling the Gpyro pyrolysis model with the optimized chemical reaction parameters, moisture and changed volume in OpenFOAM. Eventually, the predicted equation of merge thickness at various external heat fluxes is obtained, which is basically the same with that of thermal thickness.