Abstract: In order to investigate the hydrates formation and plugging in mixed transmission pipelines of oil and gas, a high pressure test loop is designed specially where natural gas hydrates are formed in diesel oil + water system with a water cut of 70% and an initial liquid flow rate of 2 000 kg/h. The experimental results show that hydrates form simultaneously in the whole pipeline, but the initial formation quantities are different. Tube wall and liquid bulk phase are the two places where hydrates mainly form at the experimental water cut. Without emulsifier, the oil/water emulsion separates at low flow rates. Hydrate particles distribute mainly in the lower liquid phase, which results in an incredible increase in the viscosity and pressure drop. Liquid stratification and the sharp increase in viscosity are the governing reason of plugging.