The invention discloses a stress concentration and ultrasonic guided wave based composite stay cable rusting monitoring method. The method comprises the following steps: manufacturing corrosion sensing units, and connecting the rusting sensing units with an auxiliary frame to form a prestress self-balance system; judging initial rusting time of a steel wire or a steel strand in the stay cable according to the data monitored at continuous stress measuring points which are distributed at different heights of the steel wire slice, and carrying out corresponding programs to calculate rusting extent, rusting rate or depth of stress rusting cracks; analyzing ultrasonic guided wave signal characteristic variation to judge the initial rusting time of the steel wire or the steel strand; comparing change rate of wave signal amplitude differences at different moments, and judging the rusting rate of the steel wire slice. The method can be used for quantitatively monitoring for a long time or regularly detecting the rusting and stress corrosion situation of the steel wires at the lower anchor region of the free section of the stay cable without damage to the local microenvironment; meanwhile, the rusting sensing units are arranged in the area, so that the monitoring authenticity and reliability can be further ensured.