Abstract:  To solve the problem of transverse cracks appearing in the inner fillet weld between the rod tube and Q460ND low-alloy high-strength steel thick-walled flange of the tertiary structural pole of a 330 kV transmission line after welding, on-site replica metallographic examination, hardness testing, residual stress measurement, spectral chemical composition analysis and crack susceptibility calculation using the carbon equivalent theory were adopted to systematically compare and analyze welded joints with and without cracks. After evaluating structural restraint, welding procedure specifications and environmental meteorological factors during welding, it is identified that the cracks are delayed cracks. Crack formation results from the combined effects of material, structure, welding process, stress and environmental conditions. Given the material and structural conditions, the fundamental cause of crack generation is that flame preheating before welding fails to meet the preheating requirements specified in the procedure. Optimized welding processes and crack repair schemes suitable for thick-walled components with high restraint are proposed, which focus on properly increasing and uniformly maintaining the preheating temperature, increasing heat input, adopting hammering stress relief, implementing post-weld dehydrogenation treatment and stress-relief tempering. These crack repair methods have been proved effective in engineering practice. The research can provide engineering references for the prevention and control of welding cracks in similar high-strength steel thick-walled structures.