Prestressed cable has been widely used in geotechnical engineering；and much attention has been paid to its reinforcing and failure mechanism. Failure mechanism of prestressed cable varies with its type and structure. One of the failure mechanisms is discussed in this paper. It is thought that separated mortar blocks surrounded by steel wire fail in shear one by one. Based on limit equilibrium method，stability judgement of blocks and recursion formula for the transmission of anchoring force after block failure one by one are obtained. The results obtained are of some significance to design and mechanism study of prestressed cable.