The welding quality of bogie products is pretty high,and the welding defect exceeding the standard,especially crack,is not allowed in any welding quality grade. Some crack develops immediately after welding, others comes out after a period of time after welding. Therefore,effective method should be taken to inhibit the generation of crack when formulating the welding process of the bogie welding parts,and preheating is the most effective way to prevent cold cracks. This paper takes a typical welded joint of bogie frame as an example and introduces how to determine the preheating temperature.