Abstract: The author first conducts a stress analysis on the triaxial specimen of reinforced soil with circular cylindrical soil bags, and believes that the soil is in a limit equilibrium state when it fails. Therefore, the strength expression of the reinforced soil specimen is obtained, and a detailed discussion and analysis of this expression are conducted. The research has shown that after reinforcement with soil bags, the cohesive force of the constrained soil is greatly improved. If the reinforcement effect of the roots after the growth of grass seeds in the bag is considered, a secondary composite reinforcement body is formed. When the roots are relatively soft, the cohesive force of the reinforced soil is greatly improved. When the roots are relatively hard, the cohesive force and internal friction angle of the reinforced soil are greatly improved. Finally, the author used the obtained strength formula for theoretical calculations and found that when the reduction coefficient was between 10% ~20%, the calculated strength line matched the experimental strength line quite well, with a maximum error of within 10%.