With rapid urbanization, underground pipeline systems are becoming more and more complex, and the risks of underground pipeline systems have increased significantly. The number of underground gas pipeline accidents in China is high, and the casualty consequences of their accidents are much higher than those of other underground pipelines. Risks posed by other underground pipelines to the underground gas pipeline system during operation are focused by those in the gas industry. In order to perform a qualitative, quantitative and systematic risk assessment of underground pipeline systems, this paper integrates two system analysis methods, STAMP and BN, to establish a STAMP-BN model, to identify system risks and causal factors based on the STAMP model, and to perform a quantitative risk assessment using BN modeling based on STAMP results and safety control structures. The model of STAMP-BN was applied to underground pipeline risk assessment, the results show that the main intermediate cause of underground gas pipeline leakage is the earth moving operation of the third-party company, and reveal the root cause of the vulnerability of the third-party company and the gas company. It is concluded that strengthening the third-party company's operational level coordination and the gas company's ability to coordinate arrangements, incident response, and identification of chronic hazards posed to other underground pipelines is key to risk mitigation.