Abstract: The overall heat transfer coefficient is a very important parameter during the operation of the buried hot oil pipeline. Whether its value is reasonable or not will influence the security and economy of the pipeline. Least square method is used to calculate the overall heat transfer coefficient for the stable operation, and a double-coupling mathematic model is presented to calculate the overall heat transfer coefficient for the unstable operation of the pipeline. On the basis of the analysis of the examples, the overall heat transfer coefficient is quantified. Besides, the model and method in this paper can be regarded to some extent as an effective reference for the unstable optimal operation of the pipeline.