Abstract: During the preheating commissioning and start-up after shutdown of submarine pipeline transporting high solidifying point oil, preheat timing as the "dominant" logo of adequate preheating pipeline temperature field is of great significance. According to the buried submarine insulated pipeline, direct preheat-flushing calculation model is established. Taking a pipeline as the study object, transient process of linear temperature in the process of preheat-flushing is studied. And then the selection of preheat timing and the influence factor are discussed. The results show that the minimum oil temperature appeared at the end of the pipe during the transient process of pipeline flushing-stabilization. Therefore, as long as the outlet temperature of the pipeline meets the needs of safe flow, the oil temperature of the whole pipe is certainly in the allowed range of flow security. The temperature requirement of crude oil safe flow is satisfied, when the preheat timing is taken at the moment of the finish temperature is equaled to the freezing point. And it is applied to different working conditions of hot water flow and flushing flow, and recommended for project.