Abstract: Bagasse Kraft black liquor lignin was used as raw material to prepare lignin nanoparticles(LNPs) which was stably dispersed in water. Then, LNPs were used as reductant and stabilizer to reduce Ag⁺ into Ag nanoparticles(AgNPs) under ambient solar light irradiation. The optimal reaction conditions for the synthesis of AgNPs were the concentration of AgNO₃:25 mmol/L, the mass concentration of LNPs suspension:100 g/L, and the solar light irradiation time of 10 min with the solar intensity of (900±100) W/m². Under the optimal conditions, the average size of the synthesized AgNPs was 15.3 nm. HRTEM and element mapping analysis showed that the synthesized AgNPs were entangled by lignin. The electronegative groups of lignin provided AgNPs an abundant negative surface charge with the Zata potential of -24 mV.The electronic doubly layer repulsion hindered the aggregation of AgNPs and stabilized AgNPs in water.