A long-range phase-sensitive optical time-domain reflectometry is proposed and experimentally demonstrated, based on the gated Raman amplification (RA). This technique can reduce the amplified spontaneous emissions (ASE) noise of the Raman pump while extend the sensing distance. A direct-detection based phase-sensitive optical time-domain reflectometry (Φ-OTDR) with an operation range of 50 km and a spatial resolution of 20 m has been demonstrated. The influence of different delay time of the gated Raman pump on the Φ-OTDR system is also discussed.