Partial discharge (PD) detection is an effective method for evaluating the insulation status of power cables. In this paper, a disturbed acoustic sensing (DAS) system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) for online PD detecting is proposed and demonstrated. To detect the ultra-small fiber strain induced by PD, a weak fiber Bragg grating (wFBG) array is introduced into the sensing fiber to increase signal-to-noise ratio (SNR) of the system. Besides, the sensing fiber is packaged with aluminum tape to enhance the sensitivity. Consistent PD signals in a 10 kV cross linked polyethylene (XLPE) power cable with artificial defects have been observed, and characteristics of the signals are analyzed.