Amine-modified lignin is regarded as a promising amine-functionalized solid adsorbent for carbon capture and storage, owing to its high reactivity, excellent thermal stability, and strong adsorption performance. Aminated lignin is typically derived from technical lignin through chemical modifications, with the Mannich reaction being a commonly employed method.1 However, the unavoidable condensation that occurs during the extraction of technical lignin from lignocellulosic biomass reduces the availability of reactive sites, lowering reaction efficiency and increasing energy consumption and costs.2 Here, we present a technological breakthrough for directly producing high N-density aminated lignin through in situ reactive fractionation of biomass, utilizing an aniline-formic acid organic system. Notably, our method employed aniline as a nucleophile to capture Cα carbocations during the formic acid delignification of biomass, thereby enabling high-yield in situ amination of lignin through selective α-arylation while simultaneously producing high-purity cellulose. Animated lignin with N contents ranging from 3.12% to 5.53% can be obtained by adjusting the aniline concentration and treatment intensity. Under optimal conditions (treatment at 140 °C for 2 hours with an aniline content of 10 vol%), a high yield and high N-density introducing 1 formanilide per 1.2 native lignin subunits) of ammoniated lignin, along with a high-purity cellulose fraction (97%), can be achieved simultaneously. High-purity cellulose fraction can be enzymatically hydrolyzed to achieve a glucose conversion rate of up to 95%. This approach is applicable to various types of biomass, including poplar, pine, and bamboo. Compared to the conventional multi-step methods involving chemical treatment and delignification, our approach is greener, more sustainable, and holds significant commercial potential for industrial applications and large-scale utilization.