This paper presents a constitutive model for biotreated sand, developed within the framework of thermodynamic theory, to describe its mechanical behavior under undrained shear conditions. The model incorporates a reinforcement index and a hardening index to account for bonding effects. Undrained triaxial shear tests are conducted to validate the constitutive model. The results demonstrate the model's capacity to accurately predict the undrained shear behavior of biotreated sand under various reinforcement levels and initial confining pressures. It effectively captures the evolution of deviatoric stress, pore pressure, and stress paths. Furthermore, the model accounts for energy dissipation and the degradation of inter-grain bonding during undrained shearing, providing a theoretical foundation for the engineering application of biotreated sand.