We propose a time and wavelength division multiplexing (TWDM)-assisted passive quadrature phase demodulation mechanism in this Letter. Combining wavelength division multiplexers (WDMs) with a programmable modulated grating Y-branch (MG-Y) laser, this method realizes both fast switching of discrete wavelengths and fast activation of multiple sensing paths simultaneously. Deploying it on a fiber-optic dual-cavity Fabry-Perot (F-P) ultrasound sensor array, we achieve high-precision localization of partial discharge (PD) signal sources in a two-dimensional (2D) plane with a maximum distance error of 1.53 cm and a maximum angle error of about 3.02°. This demodulation scheme can balance the relationship between sensitivity, sampling rate, and time-delay-induced errors, and provides an innovative solution for high-frequency phase demodulation applications of sensor arrays, which is especially significant for high-frequency detection in specific environments like partial discharges, industrial nondestructive testing, and so on.