Abstract: The effects of preheat on the temperature and stress fields of beryllium ring during laser brazing fusion welding are studied.A axial symmetry model and a thermal-mechanical decoupled finite elecment method (FEM) are adopted,and assuming that the energy distribution oflaser beam is Gauss distribution on the brazing seem surface.Preheat is realized by a special heat method.The rcsuhs show that the temperatureat outer surface rises,the temperature gradient decreases,the brazing seam depth increases.The residual stress near the brazing seem with plastic deformation decreases obviously and increases at the heat afftedzone.As a whole,preheat makes the distribution of brazing seem residualstress more homogenize.Comparing the residual stresses at outer surface of beryllium ring by FEM to that from x-ray stress measurement,the trendis identical.