lignin was modified by ultrasonic assisted Fenton oxidation, and high density polyethylene/lignin composites (HDPE/Lig) were prepared. The changes of functional groups and molecular weight of lignin before and after modification were analyzed, and the effects of lignin content before and after modification on mechanical properties, microstructure and rheological properties of the composites were discussed. The results show that the number average molecular weight (M _n) of lignin modified by ultrasonic-Fenton oxidation decreases from 15 034 g/mol to 6 375 g/mol, methoxy group decreases, phenolic hydroxyl group and carboxyl group increase. The tensile strength and impact strength of HDPE/Lig increase first and then decrease with the increase of lignin content. After modification, the optimum filling amount of lignin increases from 10% to 20%. After modification, the dispersity and compatibility of lignin are improved, the degree of microphase separation is reduced, and the physical forced compatibilization effect is better. The viscosity of HDPE is increased by lignin, and the viscosity, storage modulus and loss modulus of HDPE composites are decreased by ultrasonic-Fenton oxidation of lignin.