In the manufacturing process of titanium welded tube,irregular black spots appeared on the surface of some titanium welded tube,which not only reduced the yield,but also increased the production cost. In order to find out the cause of black spot formation,the black spot area was characterized and analyzed by SEM,EDS,Raman spectrometer and metallographic microscope. The results show that the carbon content in the black spot is high,and there are sharp D & G peaks in the Raman spectrum,which are carbides attached to the surface. Combined with the manufacturing process,it is considered that the black spot is caused by the grease mixed with the emulsion at the moving parts of the production line,which is carried to the surface of the titanium welded tube by the emulsion and carbonized during high temperature annealing. By controlling the amount of grease used in the moving parts,increasing the frequency of cleaning the spilled grease,setting up a filtering device for emulsion and increasing the pressure of cleaning water,the black spot problem on the surface of titanium welded tube is effectively solved.