Abstract: To fully leverage the advantages of X-band multi-frequency continuous wave radar in high-precision velocity and range measurement, it is necessary to address issues inherent in its reflective measurement mode, such as short detection range, inability to track low-speed targets, and measurement errors. This paper proposes the introduction of a transponder measurement mode. It derives the theoretical formula for the measurement model based on a double-sideband transponder signal. A prototype transponder was developed based on this double-sideband model. Field flight test results demonstrate that the transponder measurement mode can significantly improve echo signal quality and enable stable tracking of low-speed crossing targets. After further addressing engineering application challenges such as transponder miniaturization, low power consumption, conformal antenna design, and radar system adaptation, the X-band multi-frequency continuous wave radar transponder measurement technique will hold significant practical value. It can expand the appllication scenarios for this radar architecture and provide a new technique means for high-precision external measurement in test ranges.