In order to improve the unconfined compressive strength of expansive soil and reduce engineering hazards. Based on the research of lignin modified expansive soil, the influence of fiber length, fiber content and lignin content on the compressive strength of expansive soil was studied by single factor test. Based on the results of single factor test, the response surface test was carried out by BBD (BOX-Behnken design) response surface design method, and a quadratic regression model was established to study the effect of two-factor interaction on the unconfined compressive strength of expansive soil. The results show that under the action of single factor, the unconfined compressive strength of expansive soil increases first and then decreases under the action of each factor. When the two factors interact, the interaction between lignin content and polypropylene content is the best. The optimal combination obtained by the BBD response surface method is: the length of polypropylene fiber is 9 mm, the fiber content is 0.3%, and the lignin content is 3%. The average compressive strength of the parallel test group under this variable is 552.48 kPa, which is 76.97% higher than the compressive strength of the plain expansion. It can be seen that this scheme has the best effect on the unconfined compressive strength of the plain expansive soil. The research results provide a reference for the improvement of weak expansive soil in Xinxiang area, and provide a new idea for the experimental design of composite improved soil.