A quasi-distributed vibration sensing system based on a phase-sensitive optical frequency-domain reflectometer is proposed for monitoring the vibration of power transformer oil tanks. Compared with traditional accelerometers, the proposed system has high electromagnetic interference immunity. In this article, the principles of localization and vibration sensing are introduced. Then, the system layout and the vibration signal demodulation process are explained in detail. Next, the vibration sensing performance of the system is calibrated experimentally. Finally, vibration monitoring tests are carried out in a laboratory environment and on-site to verify the vibration detection capability of the system. The proposed system can be used to measure the time domain, frequency domain, and spatial distribution of oil tank vibration, which can be used for the online monitoring of the mechanical state of power transformers.