The widely used anchor cable in slope reinforcement usually encounter prestress loss owing to the interface failure between the cable and the cement (e.g grout), accurate estimation of pretress loss in anchor cables is of great significant to the stability of slopes. In view of the limitation of the existing anchor cable-grout decoupling model, a three-stage linear function was applied to demonstrate the relationship of shearing stress and displacement at the interface of cable and grout, to modify the collateral resistance distribution at the interface after decoupling. Consequently, the decoupling segment was refined into two parts consisting of plastic element and residual element, and an anchor cable-grout decoupling model was proposed based on this refined slip-shear relationship. Then, the in-situ long-term monitoring was conducted for anchor cables embedded in a high rocky slope under construction in Shenzhen city to provide real monitoring data of prestress change, by which, the proposed theoretical model for prestress loss estimation was verified. Furthermore, the 3-D finite element model of the target slope was established and a numerical analysis method was proposed for simulating anchor cable-grout decoupling. As a consequence, the verification among the theoretical model, real measuring data and numerical simulation was accomplished. The results show that the prestress loss calculated by the proposed model is 22%, and the real measured and numerical simulated ones are 15.8% and 14%, respectively. The small differences indicate that the proposed theoretical model and simulation method are reasonable. The research results provide a reference for accurate estimation of prestress loss of anchor cables in slopes.