In order to explore the effect of lignin on weak expansive soil in Xinxiang area, the physical and mechanical properties of weak expansive soil in Xinxiang area with different lignin contents were examined by laboratory test and ESEM (Environmental Scanning Electron Microscope) test. The microstructure characteristics and pore morphology characteristics of the improved soil with different lignin contents were qualitatively described and quantitatively characterized. Based on the XRD ( X-Ray Diffraction ) testing results, the changes of mineral composition in the improved soil were discussed, so as to reveal the interaction mechanism between lignin and soil. The test results show that when lignin is used to improve expansive soil, 3% lignin has the optimal effect on inhibiting the expansion, and simultaneously the compressive strength reaches the maximum, while the compressive strength of soil decreases after exceeding the optimal dosage. The pore structure in the soil is complex, the pore arrangement is disordered and the orientation is less satisfactory. The lignin cement can fill the soil pores to make its microstructure more dense. Lignin plays a cementing role in expansive soil, and cannot react with expansive soil to form new mineral components. It has satisfactory stability when interacting with soil, which belongs to physical improvement. As an efficient and environmentally friendly modifier, lignin can effectively improve the basic engineering properties of expansive soil, of which further study is promising.