Abstract: The residual stresses in a laser welded beryllium ring were measured using neutron diffraction stress instrument on a stable reactor. Firstly, the parameters of neutron diffraction stress measurement for beryllium were studied and optimized. Keeping the reactor power, diffraction plane and gauge volume fixed, the neutron diffraction curves at a point in the beryllium ring were measured repeatedly by using the measure time of 600 s and 6 000 s, then the statistical errors for the strains of beryllium were deduced, which for the later is only 1/3 of the former, and the standard error is about 1.10×10^-5. The neutron diffraction curves along axial strain and hoop strain near the weld line of the beryllium ring were measured using the optimized time 6 000 s. Both of the residual strains in the beryllium ring were obtained, and the residual stresses were also calculated. The temperature and stress fields in the beryllium ring during laser welding process were simulated by using a 3D finite element model. The computed residual stress and strain at the middle of thick wall in the beryllium ring were compared to those by neutron diffraction, which shows they have an identical trend.