Abstract: Aiming at the problem of post construction detection of anchor cable prestress in existing slope under complex geological conditions, the traditional method is difficult to achieve efficient and accurate detection. It is urgent to develop new detection technology to evaluate the effective prestress state of anchor cable, so as to provide scientific basis for slope stability analysis, maintenance and reinforcement. In this paper, the prestress detection technology based on the single anchor cable reverse pulling method is proposed, and an improved servo type prestress intelligent detection system is developed. The accuracy and repeatability of the detection system were verified by laboratory tests. Then, combined with a slope engineering case of an operating expressway, 14 anchor cables were tested and applied in the field. Laboratory tests show that the detection technology has good detection accuracy and repeatability. The field test results show that only 2 of the 14 anchor cables have effective prestress to meet the design requirements, and the remaining 12 cables have effective prestress less than 80% of the design value, with a minimum of 2%, revealing that the existing anchor cables generally have problems of corrosion and load attenuation. The single anchor cable reverse pulling method can accurately evaluate the effective prestress state of the anchor cable after construction, which provides reliable data support for slope stability analysis and reinforcement design, and verifies the engineering applicability of the prestress compensation technology. The research results provide effective technical support for the long-term safety monitoring and maintenance of slope anchorage engineering.