In order to explore the effect of lignin on weak expansive soil in Xinxiang area, the physical and mechanical properties, microstructure and pore characteristics, mineral composition changes and the interaction mechanism of improved expansive soil with lignin were discussed, by laboratory test, ESEM ( Environmental Scanning Electron Microscope ) test and XRD ( X-ray diffraction ) test. The test results show that when lignin is used to improve expansive soil, 3% lignin had the optimal effect on inhibiting the expansion of expansive soil, and the compressive strength of soil is the highest at this time. The pore structure in soil is extraordinary complex, and the pore arrangement orientation is poor. Lignin cement can fill the pores of soil to make its microstructure more dense. Lignin plays a cementation role in expansive soil, and can not react with expansive soil to form new mineral components. It has satisfactory stability when interacting with soil and belongs to physical improvement.