The invention provides a method for measuring internal 
residual stress of a 
thick plate welding part. The method is based on an inherent strain method, the 
welding inherent strain is decomposed, and an original 
welding sample is 
cut into simple samples in different forms; measuring 
residual stress values of the simple samples by adopting an X-
ray diffraction method, and then inversely calculating inherent 
strain distribution conditions of the simple samples by adopting a 
finite element method; 
finite element solution is carried out on the internal 
residual stress by applying the inherent strain. Comprising the following steps: decomposing inherent 
strain distribution according to distribution characteristics of inherent strain, 
cutting a welding part, and separating out a plurality of different samples; the 
cut surface is treated, and the residual stress of the 
cut surface is measured through an X-
ray diffraction method; establishing a sample two-dimensional inherent strain calculation model according to a measurement result, and inversely calculating the inherent strain; finally, an original 
thick plate welding part internal residual stress calculation model is established, the inherent strain acts on the model, 
thick plate welding part internal residual 
stress distribution is obtained, the measurement result is reliable, and efficiency is high.