Long straight string beams are often used as supporting structures in large equipment. Due to the large size of string beams, welding technology is generally used to complete their production. This article first constructs the finite element model and heat source model of the chord beam based on the structure of the long straight beam, and then obtains the distribution and evolution law of the temperature field, as well as the temperature rise law of the temperature field under dynamic simulation. Based on this law data, an improved BP neural network model is used to predict the deformation of the beam. In order to verify the authenticity of the prediction results, the article finally builds a Matlab simulation experimental environment and conducts simulation experiments on the welding deformation prediction of the beam.