Abstract: The distribution of the primary air (PA) along the grate length plays a vital role in the combustion characteristics of the biomass-fired grate boiler, but there is no relevant report at current stage. In this paper, the biomass-fired grate boiler with an evaporation capacity of 130 t/h is selected, and the uniform (baseline), staged and refined-staged PA distribution cases are numerically simulated to comprehensively investigate the combustion and pollutants formation performance when the different PA distribution cases are conducted. The results show that: 1) The relative errors between the simulations of the baseline and the several measured results of the field are all within 10%. 2) Compared with the baseline, the oxygen content is increased during the volatile and char combustion process of the staged and refined-staged cases, promoting the burnout of the two matters. As the result, the char burnout ratio of the latter two cases remarkably increases from 76.5% to 87.8% and 95.5%, respectively. Correspondingly, the amount of the combustibles released from the solid phase to the freeboard is reduced, resulting in lower flue gas temperature peak and more uniform temperature distribution. 3) The local high temperature and oxygen-rich regions are avoided in the latter two cases, which is beneficial to suppress the formation of NO_x, thereby reducing the NO concentration at the outlet decreases from 172 mg/m³ (11% O₂) to 161 mg/m³ and 156 mg/m³, respectively. In summary, implementing staged or refined- staged primary air distribution along the grate length significantly improves combustion and pollutant generation characteristics of biomass.