[Objective] To construct a constitutive model suitable for the vetiver grass-soil composite at different growth stages, for the precise simulation and prediction of its mechanical behavior. [Method] This study selected vetiver grass-lateritic soil composites at three growth stages (20 days, 40 days, and 60 days) as the research subjects, and mechanical data were obtained through triaxial tests. Based on the mixture rule, the Duncan-Chang model and the linear elastic model were employed to calculate the constitutive relationships of the soil and plant roots, respectively, and these were superimposed according to the volume fraction. [Results] (1) The two-phase superimposed constitutive model constructed based on the mixture theory demonstrated excellent performance in simulating the mechanical behavior of the root-soil composite (R2 > 0.89). The stress-strain curves of the model accurately reflected the shear strength of the root-soil composite in triaxial tests and the variation trends of stress-strain relati