Abstract: After restarting a radar, the ranging system error witnesses a significant variation (around ±0.8 m), becoming the primary source of error in radar ranging. Analysis reveals that the clock multiphase issue after frequency division by the clock division circuits in the digitial transceiver module is the root cause of ranging system error. To address the common issue prevalent in this type of radar equipment, a power-on self-calibration method is proposed in this paper. After the restart of radar, the offset error of ranging system caused by clock multiphase is on-line collected in real-time, and the time-variant error component is compensated and subtracted, so as to achieve a stable ranging systematic error. After applying this calibration scheme, error in radar ranging is significantly improved, and high ranging accuracy is achieved in both static and dynamic field flight tests, laying a solid foundation for meeting the requirements of ultra-high measurement accuracy for carrier-based unmanned aerial vehicle landings. This calibration method is simple to operate, fast to execute, neither requiring modification to the hardware circuits, nor relying on external auxiliary measurement equipment. Moreover, it showcases remarkable optimization effect and has substantial reference value for radar systems with intermediate-frequency self-loop links or fully-coherent radar systems with radio-frequency self-loop links.