Abstract: A mockup-to-production method was proposed to investigate the welding residual stress induced by the welding of reactor pressure vessel（RPV） head and control rod drive mechanism（CRDM） nozzle. The mockup was built firstly, the welding temperature and residual stress were measured from the mockup; then the finite element simulation on the mockup was carried out and the simulated temperature and stress results were validated by the experimental data; based on the mockup, the simulation model and algorithms were further optimized and were finally used to simulate the residual stress of the product. The residual stress in the nuclear RPV head with two J-type welds at the locations of non-center hole were efficiently simulated with the proposed method. In addition, the superposition effect at the region between the two J-type welds were investigated. Results show that the mockup-to-production method is an efficient and effective method for the simulation of large nuclear welded structures; the superposition effect at the region between two J-type welds in RPV head is not significant.