[Objective] To establish a constitutive model applicable to different growth stages of vetiver grass root-soil composite，in order to precisely simulate and predict its mechanical behavior. [Methods] Three groups of vetiver-laterite composite samples at different growth stages（20 days，40 days，and 60 days）were selected as the research subjects. Mechanical parameters were obtained through triaxial tests. Based on the mixture rule，the constitutive relationships of soil and plant roots were calculated using the Duncan-Chang model and the linear elastic model，respectively，and then superimposed based on volume fraction. [Results] 1）The two-phase superimposed constitutive model based on the mixture rule demonstrated excellent performance in simulating the mechanical behavior of the root-soil composite（R² >0.89）. The stress-strain curves of this model accurately reflected the changes in the shear strength and stress-strain relationship of the root-soil composite in the triaxial tests at different growth stages. 2）Using the Duncan-Chang model and the linear elastic model as the constitutive relationships for the primary and reinforcement phase，respectively，the characteristics of the stress-strain response of the two materials during shear could be effectively simulated and accurately reflected. 3）The vetiver grass roots effectively reinforced the soil. During the growth cycle，with the extension of time，the number，diameter，and volume of the roots gradually increased，while the elastic modulus gradually decreased. The reinforcement effect of the roots progressively enhanced，leading to a gradual increase in the shear strength of the root-soil composite structure. [Conclusion] The proposed model not only comprehensively describes the stress-strain relationship of the root-soil composite but also accurately evaluates the soil reinforcement effect of plant roots at different growth stages through parameters such as root quantity，single-root elastic modulus，and root volume. The results provide a scientific basis for slope stability analysis and the application and planning of ecological slope protection technologies.