Abstract: Natural gas flow measurement loop system can reduce the dependences on upstream natural gas supply, gas flow and gas pressure at exhaust channel of calibration station. Meanwhile, it can expand test capacity and further improve the traceability system of gas flow quantity. For the loop system, the key technical indicators of process and control are required highly, and the measurement uncertainty is affected by various factors. In this article, the domestic and foreign technical indicators and the characteristics of gas flow measurement loop system are introduced. The natural gas loop system of Chengdu Branch, and the influence factors of measurement uncertainty are analyzed. In addition, the measures and suggestions on reducing the overall measurement uncertainty of loop system are present.