Biomass pyrolysis can generate tar and gas products with high industrial value. But the nitrogen (N) element in the biomass can inevitably migrate to the products along with the pyrolysis process, thus possibly polluting the environment. Focusing on the overall goal of preparing clean energy from biomass resources, this study systematically analyzes nitrogen migration and conversion mechanism during biomass pyrolysis, focusing on the research progress of the generation and conversion mechanism of gas nitrogen, tar nitrogen and char nitrogen. The NOx precursors can be the HCN and NH3 in the biomass pyrolysis gas. Specifically, the NH3 comes from the amino acids that are released from the amino acid pyrolysis and hydrolysis of HCN on the surface of char, while the HCN is from the secondary cracking of primary pyrolysis products, such as nitrile and N-containing heterocycle. The N-containing substances in the pyrolysis oil include the N-containing heterocycles, nitrile, and amide. Furthermore, the N-containing heterocycles can be produced by the fragmentation of some amino acids and by dehydration condensation between the amino acids. The nitrile is derived from the de-H2 reaction of amino acid molecules and the de-H2O reaction of amides. The substitution reactions can also be used to form amides from NH3 and carboxyl groups. More importantly, the biomass varies greatly in the different pyrolysis characteristics and products, due to the composition during the reaction. The higher heating rates can promote tar cracking for higher NOx precursor production during biomass pyrolysis, while the lower heating rates can contribute to tar production for better quality. The pyrolysis temperature and atmospheres of biomass can pose a large effect on the yield and composition of the pyrolysis products. The pyrolysis in the O2 and H2O atmosphere can enhance the conversion of HCN to NH3, while the pyrolysis in the CO2 atmosphere can reduce the production of NOx precursors. In terms of the pyrolysis pressure, the gas-N residence time can facilitate the reaction path of the secondary pyrolysis for the migration path of nitrogen. The larger particle sizes of the biomass can increase the NOx precursors but less the tar production, whereas, the smaller particle sizes can promote the N fixation in the char. The catalysts can reduce the pyrolysis time and the temperature for the N migration and conversion during biomass pyrolysis. The mineral elements (such as K, Ca, and Fe) in the biomass can promote the conversion of nitrogenous substances in the coke into the HCN. By contrast, the metal oxides (such as Fe2O3, Co3O4, and NiO) can be used to enhance the production of Tar-N, where Co3O4 has the best performance. The KOH can reduce the types of hydrocarbon compounds in the pyrolysis oil, but for less NH3 and HCN production. The current NOx treatments are the catalytic, plasma, microbial, absorption, and adsorption methods. All tail-end treatments cannot reduce the emission of pollutants with low efficiency and high energy consumption. Anyway, the N migration and transformation mechanism in the pyrolysis of biomass can reduce the emission of N-containing pollutants at the source during the pyrolysis process.