Pressure-distributed unbonded prestressing anchor cable is the major support for the strongly unloading rock mass in the left-bank high and steep slope of Jinping Hydropower Station. Large anchoring force and small spacing of the anchorage gave rise to notable compressional deformation of the rock mass; the mutual influence of anchor cables led to massive prestress loss. A physical model test on the anchoring of unloading rock mass was carried out to examine the displacement and deformation of rock mass as well as the superposition of anchoring force and prestress loss in association with Mindlin’s solution. The anchor group effect mainly includes 1) the compression of unloading rock mass posed by stress superposition; 2) the prestress loss of anchor cable caused by tensions on adjacent ones. Furthermore, the calculation methods of rock mass displacement and prestress loss rate were obtained via experimental analysis and theoretical derivation. Reasonable spacing of anchor cables and appropriate increase of anchor cable length can effectively reduce the prestress loss caused by group anchor effect.