With the increasing emphasis on environmental protection and sustainable economic development, recycling industrial by-products for soil improvement has received increasing attention in geotechnical engineering. Lignin fiber, a by-product of the paper industry, has the advantages of good flexibility and dispersion, etc. Given these advantages of lignin fiber, this paper uses lignin fiber to improve expansive soils' strength and swell-shrink characteristics, then carries out a series of experiments to evaluate the effect of the improvement. Soil specimens with 0%, 1%, 2%, and 4% doped lignocellulosic fibers were prepared indoors, and these specimens were subjected to the unconfined compressive strength test, the consolidated undrained triaxial shear test, the unloaded expansion rate test, and the shrinkage test. The mechanism of lignin fiber's action in improving expansive soil was revealed by X-ray diffraction test (XRD) and scanning electron microscope test (SEM). XRD and SEM tests have shown that lignin fibers act as a ''bridge'' lap and fiber web in the soil. The ''bridge'' lap connects the soil particles and enhances the connecting force between the soil particles. The fiber mesh gives the soil a good stress structure and limits the sliding of the soil particles to a certain extent. The value of unconfined compressive strength of expansive soils is maximum under 2% content of lignin fibers with an increase of 54%. Under 4% content of lignin fibers, the expansion soil had the least unloaded expansion rate, which was reduced by 32.8%. Combining all the test results, it was obtained that expansive soil was best modified with 2% lignin fiber content. In conclusion, using lignin fiber as an additive to modified expansive soil is viable and can lead to resource recycling.Graphical AbstractAdequate amount of lignin fiber, in the soil body can play a bridge lap and fiber web role. The bridge lap can connect the soil particles together, enhance the connection between soil particles, so that the soil body to form a stable structure. The fiber web gives the soil body a good stress structure, which can evenly spread the load to other regions of the soil body. The fiber web restricts the sliding of the soil particles to a certain extent, which can enhance the friction of the soil body. However, Excessive lignin fibers tend to collect into clusters within the soil body, separating the soil particles, which tends to form weak surfaces within the soil body, and the presence of weak surfaces affects the overall stability of the soil body.