Quantitative demodulation of the external vibration is demonstrated with a direct-detection based Phase-sensitive OTDR system. An acousto-optic modulator is utilized for both probe pulse generation, and optical phase shifting simultaneously. The fiber under test is probed with a phase shifted double pulse pair scheme, with the phase difference between the two pulses periodically shifted with a π/2 step. The optical phase information is retrieved using the homodyne inter-pulse interference signal with direct detection only. Experimental results show good consistence between the phase demodulation result and the applied vibration amplitude under a cost-effective system configuration.