Abstract: After short operation of the 660 MW ultra-supercritical unit of a thermal power plant, the nozzle socket of the drain pipe cracked in the hot section, causing steam leakage. The chemical composition, hardness and microstructure of the nozzle socket were analyzed. A three-dimensional solid model was established according to the actual layout of the pipe, and the force of the structure was simulated and analyzed by the thermo-solid coupling finite element method. The results show that the chemical composition, hardness, and microstructure of the nozzle socket met the requirements. The maximum stress obtained by the finite element simulation was located at the lower edge of the connection weld between the nozzle socket and the main pipe, which was consistent with the actual cracking position. The cracking was due to the limited ability of the drain pipe to absorb deformation, and the higher tensile stresses in the nozzle socket area induced by the blocked thermal expansion of the pipe because of the unreasonable pipe layout. After adding a U-shaped expansion bend to enhance the flexibility of the pipe and reduce the structural stress, the nozzle socket of the drain pipe did not crack during operation for about 1 a.