The research of shutdown temperature drop rule in heated oil pipeline is the primary condition to ensure the safe restarts of the pipeline. Because of the complicated geological conditions along the pipelines, the pipelines often cross rivers and lakes. But there is no thermal-storing material such as soil around the underwater pipelines, so the shutdown and restart time of the whole lines depends on the temperature drop of the underwater pipelines. With the development of offshore oil field, the research of safe shutdown and restart rule become very important. Using FLUENT software, enthalpy-porosity technique was adopted to simulate the temperature drop rule of under water line during stopping transportation considering the rude oil concretion latent heat effects in temperature drop, and the position of erstarrungs zone and dilution zone and liquid oil zone was gained in different time. The result indicates that the temperature of rude oil in pipeline drops quickly at the beginning of stopping transportation; in the middle and later period, because the rude oil solidification release thermal and the layer of solidification become thicker, thermal resistance becomes larger, and the rate of temperature drop is decreased. The simulate result is in accordance with the fact.