This study proposes an online monitoring technology for oil and gas pipeline leakage based on a high-fidelity fiber-optic distributed acoustic sensor (HiFi-DAS) and introduces the measurement principle and technical advantages of DAS. The DAS can successfully detect and locate the leakage in a gas pipeline in a simulated field experiment. Considering the complex field environment and strong background noise, the wavelet denoising algorithm is adopted to further improve the detection sensitivity of leakage events, and gas leakage under pressure of 0.05 MPa is accurately detected in field experiments. Experimental results show that the proposed DAS system has superior performance in the field of oil and gas pipeline safety monitoring.