A reactive compatibilizer containing flexible segment (RCFS) was synthesized and characterized by Fouriertransform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC). The RCFS was used as acompatibilizer for the preparation of lignin/polypropylene (PP) wood plastic composites, and the effect of the RCFSon the properties of composites (e.g., mechanical properties, thermal properties, crystal structure, and morphology)were investigated in detail. Experimental results indicated that the lignin/PP composite displayed the best performancewith the addition of 2 wt% RCFS. Its impact strength, flexural strength, and modulus were 56.8%, 37.3%,and 20.6%, respectively, which are higher than those of the untreated composites. Moreover, RCFS treatment couldalso increase the thermal stability, crystallization rate, and degree of crystallinity of lignin/PP composites.