Real-Time vibration localization and phase demodulation at a low system cost are still the challenges for phase-sensitive optical time-domain reflectometry (Phi-OTDR). In this paper, undersampling technique is utilized to decrease the data volume, also contributing to the reduction of the system cost. Digital envelope extraction is applied to quickly locate the vibration, and phase demodulation with the undersampled data is also performed for waveform analysis. This flexible scheme improves the real-Time performance of the system, also offering an enhancement to the applicability of Phi-OTDR.