Prestressed cables are widely used in reinforced concrete work of geotechnical engineering. In this paper, prestress change laws and factors influencing prestress change are studied systematically, and a model predicting the prestress loss is built based on the coupling of rock mass creep and relaxation of prestressed cable. This suggested model can predict prestress loss and the final prestress of prestressed cable, and is proved from an engineering illustration that calculated values obtained by authors formula rather approach to measured data and their change laws are similar. 