Abstract: Laser welding was used to conduct welding experiments on the molybdenum tube with wall thickness of 0.5mm and the molybdenum rod as an end plug. With the aid of an optical microscope（OM）, scanning electron microscope（SEM）and tensile testing machine, the effect of preheating temperature on the structure and properties of the welded joints were studied. The results show that with the preheating temperature increasing, the grain size of the weld melting zone gradually increases, and the oxygen content in the weld zone first decreases and then increases; when the preheat temperature is 400 ℃,the oxygen content is the lowest; the strength of the welded joint first increases and then decreases, and also reaches the highest when the preheating temperature is 400 ℃. The laser welded joints under different preheating temperatures are all intergranular brittle fracture along the weld centerline during tensile test. Preheating reduces the stress of welding, improves the distribution of oxygen element in the joint and reduces the brittleness of the joint, thereby effectively improves the strength of the welded joint.