A three dimensional transient model was developed to simulate the pyrolysis process of low-rank sub-bituminous coal in a laboratory-scale externally heated fixed-bed reactor. The water evaporation and the release of volatiles were taken into account in the model. The simulation was first validated by the central temperature evolution of reactor. The evolution of bed temperature, generation of gaseous products, variation of voidage and its influence on the flow behavior of gaseous products were further analyzed. The obtained results show that the phase change of moisture determined the heating process, delaying the temperature of inner coal zone increase above the boiling temperature and consequently the start of pyrolysis. The non-uniform radial distribution of voidage led to the phenomenon that gaseous products generated in the inner low temperature zone tended to flow radially towards heating wall before flowing out the reactor. Analyses indicate that in the temperature range of intense volatiles release the dominant heat transfer mechanism was radiation.