Abstract: Conventional analog I/Q demodulator suffers from phase and amplitude imbalance and DC offset, which cause big error into the measurement. A digital PAD is designed. Based on I/Q demodulation principle, using digital algorithms, such as digital filter and Hilbert transform, the conventional measurement error can basically be removed. Measuremental results show that the digital PAD has a maximum phase error of ±0.5° and a resolution of 0.2°. Its temperature coefficient is -0.1°/℃. Its dynamic ranges for phase-measurement and amplitude-measurement are -18~5 dBm and -20~0 dBm, respectively. The digital PAD can meet the need of the BEPCⅡ phasing system.