A mockup-to-production method was proposed to simulate the residual stress in multi-pass welding of large structures.Firstly,the mockup was manufactured and tested to obtain data such as temperature cycle and residual stress. Then,the numerical calculation of the residual stress of the mockup was carried out. The model and algorithm of the mockup were calibrated by the test data,and a series of optimization and efficient calculation were carried out,such as the simplification of the weld path. Finally,the optimization algorithm and model were used to simulate the residual stress of large components. The residual stress in the nuclear reactor pressure vessel（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.