There are a lot of pipelines whose heat loss exceeds the limit of national standards after they are used a period of time in refining pipeline insulation engineering. Thermal defects are easily found on the pipeline surface by using a infrared thermal imager.There are two kinds of thermal defects on these pipelines. One is settlement of insulation layer; the other is seam in lap joints of insulation protection casing. Numerical simulation is developed in this paper for these two cases. Heat loss and surface temperature of pipelines are calculated to verify if the value has exceeded the limit of national standards. Some improving suggestions are also given in this paper.