[Objective] A nonlinear failure criterion of root-soil composites and the nonlinear characteristics of root-soil composites was studied combining with the strength of root-soil composites under three root content conditions (G=0.12%, 0.24%, and 0.36%) and three moisture content conditions (w=13.22%, 16.22%, and 19.22%). [Methods] The GDS triaxial apparatus was used to conduct consolidated-undrained triaxial tests to obtain the stress-strain curves and shear strength parameters of root-soil composites. [Results] Unreinforced soil and root-soil composite stress-strain curves exhibited characteristics of hardening. Additionally, moisture content was negatively correlated with the peak stress of rooted soil, while root content was positively correlated. Under the condition of G=0.36%, 70.1% was the maximum increase of peak strength of the root-soil composite. When w=13.22%, the peak strength of the root-soil composite increased by a maximum of 86.7%. The nonlinear failure criterion of root-soil composites was nonlinear when the normal stress was lower than the critical stress, and linear when the normal stress was higher than the critical stress. The key parameters that described the effect of roots in the nonlinear failure criterion were c and k. c had a positive correlation with root content and moisture content. The contribution of roots in rooted soil shear strength increased with increasing c. Moreover, k was positively correlated with root content and negatively correlated with moisture content. The smaller the value of k, the more obvious the nonlinear characteristic of the strength envelope of the root-soil composite. [Conclusion] The strength of root-soil composites is greatly influenced by root content and moisture content. The stress-strain relationship curves of both unreinforced soil and root-soil composite showed a hardening type, and the hardening degree of the root soil composite is significantly higher than that of unreinforced soil, indicating that the addition of roots can simultaneously increase the strength and stiffness of the soil.