A large-capacity, long-distance distributed acoustic sensing (DAS) system without inline optical amplification was proposed and experimentally demonstrated using an ultra-weak fiber Bragg grating (UWFBG) array and coherent detection. The effect of the finite extinction ratio of an acousto-optic modulator and the Stokes signal of stimulated Brillouin scattering (SBS) in UWFBGs on the performance of DAS was simulated and revealed. A high extinction ratio and a balanced input pulsed optical power can improve the capacity and distance of the DAS. The dynamic acoustic signal can be well reconstructed for a serial array of 10828 near-identical UWFBG with a length of 54.14 km. An acoustic signal sensitivity of 189.54 pɛ/√Hz and a signal SNR of 40.01 dB with a spatial resolution of 5 m can be achieved at the far end.