Abstract: The ground improvement using plants is prone to take into account the stability of the soil and ecosystem functions, however, it is difficult to achieve an ideal effect at the initial stage of plant growth when both the number of roots and the length are not sufficient. Therefore, a combined plant-biopolymer reinforcing method is proposed. The viscous solution of biopolymers can improve the soil strength at the initial stage of plant growth, and meanwhile provide carbon, nitrogen and other nutrients needed for plant growth. To this end, a two-stage experiment is designed to investigate the effects of oat roots and β-glucan on strengthening soil at the early and late stages of plant growth. The soils improved by the oat roots or β-glucan only, and untreated soil samples are also prepared for comparison. The results show that the water content of oat root-treated soils is lower than that of untreated soils due to water absorption and soil cracks caused by root squeezing. The β-glucan can improve soil electrical conductivity (fertility) and reserve water by its biopolymer hydrogels, which is conducive to plant growth. Furthermore, at both stages of plant growth, the soil treated by the combined method has the highest shear strength. On the one hand, the β-glucan can improve the soil strength by improving the bond between the soil particles at the initial stage. On the other hand, the oat roots play an important role in improving the soil strength at the late stage, while the β-glucan is decomposed and taken up by plant roots and absorbed by soil microorganisms, providing limited strengthening effect. The above results will further promote the green soil reinforcement technology and expand the applicability of plant to slope protection.