The effectiveness of reinforcing rock mass with prestressed anchor cables depends on the magnitude of prestress. To keep the rock mass stable in the long term, sufficient prestress must be maintained. Factors such as time-dependent rock deformation, anchor relaxation/corrosion and anchorage differences cause prestress loss, adversely affecting reinforcement. This paper focuses on prestress loss due to time-dependent rock deformation. The loss can be expressed as σ_L=σ_0+σ_a+…; based on the viscoelastic properties of rock, formulas for σ_a and σ_… are derived, and the influence of bond and free lengths on reducing prestress loss is discussed.