Abstract: Under the irradiation environment, the grid springs undergo creep deformation due to neutron irradiation, resulting in the degradation of spring force with increasing burnup. This paper presents the development of a grid spring relaxation analysis software, named "BEGONIA", which comprehensively considers the influence of irradiation creep of springs, cell size variation as well as fuel rod diameter variation on the elastic deformation of springs, enabling the prediction of spring force after irradiation. The verification based on the fuel rod pull-out test shows that the average relative deviation between calculated and measured post-irradiation spring force is 17.1%, and all the experimental values fall within the envelope range calculated by the software, indicating good predictive accuracy of the computational model. This software can provide reliable support for the performance evaluation and design optimization of fuel assembly grids, and has promising prospects for engineering applications.