Removal of angular deformation from welded structures induced during the welding of butt joints in thick steel plates needs high cost and expert skill. In order to reduce deformation, proper joint designs are studied with the prediction of deformation prior to welding. However, as the thickness of plate increases for a structure to gain higher strength, analysis of the welding process for the prediction become more difficult, because of the increase in number of welding passes in the joint. In this study, a numerical model with the finite element method (FEM) was developed to analyze the angular deformation in multi-pass welding of butt joints of plates made of AH32 steel that had a thickness of up to 100 mm. A series of numerical simulations were then carried out based on the developed model to predict the deformations for the thick plates. With the results obtained by the analyses, this study suggested optimal X-shape grooves for the butt joints of thick plates to minimize the angular deformation, revealing that as the thickness of the plate increased to 100 mm, the ratio of the depth of the front-side groove to that of the back-side groove should be gradually increased to nearly 1:3.