The invention discloses a method for in-situ removal of nitrogen pollutants in biomass fuel gas in a biomass gasifier. An exogenous metal-based catalyst is added into biomass in the biomass gasifier and combined with the catalytic action of endogenous mineral components of the biomass to enable the biomass to be subjected to catalytic pyrolysis, biomass nitrogen is converted into N2 in a high selectivity manner, HCN and NH3 produced by self pyrolysis of the biomass are used to reduce NOx and N2O produced by semi-coke oxidation, the biomass nitrogen is catalyzed to be converted into nitrogen in an oriented manner during the process of biomass gasification, pollution control with pollutants is realized, most of nitrogen is converted into N2 in the gasifier to the uttermost extent, content of the nitrogen pollutants in gasification crude gas is reduced from the source, efficient in-situ removal of the nitrogen pollutants inside the biomass gasifier is realized, the total removal rate of the nitrogen pollutants is 60-80%, and the method has wide popularization and use value.