Display omitted •Alkaline lignin and DM resins were utilized to modify poplar fast-growing wood.•The materials used were green and environment friendly.•The DM-AL modified wood obviously increased dimensional stability.•The DM-AL modified wood with 12 %AL displayed excellent mechanical properties.•The DM-AL modified wood was found effective against decay fungus. Fast-growing poplar wood was modified by using 1,3-dihydroxymethyl-4,5-dihydroxyethylideneurea (DM) and alkaline lignin (AL) to improve its mechanical properties and decay resistance. AL was first used in combination with DM for wood impregnation modification. The weight percent gain, dimensional stability, mechanical properties, decay resistance and microcosmic changes of all wood samples were evaluated systematically. The results indicated that the maximum weight percent gain and anti-swelling efficiency of poplar wood after modification with DM-AL reached 46.2 % and 52 %, respectively. Compared with the unmodified samples, the bending strength, modulus of elasticity and hardness of the wood samples modified with DM-AL increased by 40 % – 60 %. Notably, the impact strength of samples modified with DM-AL was 18–26 % higher than that of samples modified with DM only, which may be due to the fact that the AL macromolecules slowed down the infiltration of DM-AL into the wood cell walls. The decay resistance of optimal modified samples against the decay fungus was improved by ∼80 % compared with unmodified sample. Furthermore, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) confirmed that DM-AL modifiers impregnated into wood cell lumens. Fourier transform infrared spectroscopy (FTIR) demonstrated that the DM resin crosslinked with wood cell walls and AL macromolecules. In general, this study provides a facile, low-cost and green method to improve the comprehensive performance of fast-growing poplar, including mechanical properties and decay resistance.