The optical fiber sensing technology has the advantages of long-distance transmission and distributed detection, therefore having great application potential in the field of cable line condition monitoring. In this paper, a phase-sensitive optical time-domain reflectometry (Φ-OTDR) is used to interrogate partial discharges in the fault diagnosis of power cable joints. By virtue of stochastic features of the Rayleigh scattering coherent signal, the signal variance is chosen as a detection quantity of the weak ultrasonic signal. In the partial discharge experiment, five fiber loop sensors were wrapped around some ultrasonic monitoring points on the cable joint, to prove the distributed locating capability of the Φ-OTDR system. The signal characteristics of the electrical measurement method and fiber-optic acoustic measurement method for local discharge were compared, and the positions of the five fiber loop sensors were calibrated by a piezoelectric sensor, which verified the features of the Φ-OTDR distributed ultrasonic fiber sensors in partial discharge measurements.