After assembling and welding of a nuclear power directly-welded diaphragm,a large number of cracks were found in the heat-affected zone（HAZ） at the steam inlet and exit sides of the blade convexity by penetration inspection. In this paper,the thermalmechanical coupling calculation of this structure is carried out by SYSWELD,the crack initiation mechanism was analyzed,structural stress,transformation stress and hardened structure are the fundamental causes of delayed crack in the HAZ of the blade. It is clear that the excessive manual grinding of groove is likely to cause the penetrating crack along the thickness direction of the blade;vicarious optimized welding procedures are simulated,the results show that the structural stress,transformation stress and hardened structure are all improved to some extent,a special welding procedure adapting intermediate heat treatment,preheating,post-heating and four-stage cross welding sequence is proposed,the surface stress in the HAZ of the blade will be reduced by about 260 MPa,which will greatly lower the delayed crack tendency of the subsequent diaphragms.