A phase detection system has been implemented at the Taiwan Photon Source, employing beam position monitor (BPM) electronics integrated with a digital inphase/quadrature (I/Q) demodulation scheme. This system enables high-resolution analysis of beam phase stability, RF cavity field phase, and synchronization integrity of the RF clock distribution. It also facilitates the investigation of beam phase variations under dynamic operational conditions, including changes in insertion device gaps and booster ramping cycles. This paper presents the operational principles, implementation architecture, and representative experimental results of the developed detector system.