In this study, an improved multigroup (MG) cross section library correction system for the DeCART2D transport lattice code was developed by introducing group-wise correction factors for absorption, fission reactions, and the scattering matrix. In this system, corrected cross sections are generated by the product of the correction factor and the cross section, and the library corrections are performed through multiple iterations using the fixed-point iteration method. To examine the performance of the improved correction system, APR-1400 benchmark analyses were performed using an optimized DeCART2D MG cross section library generated by the correction system. Notably, the library produced by the improved library correction system showed better accuracy in terms of reactivity and power distribution compared to a library generated by the existing library correction system and an uncorrected library. In the near future, the newly improved MG cross section library correction system will be applied not only to the 47-group library for pressurized water reactor systems but also to MG libraries with a larger number of energy groups for Gen-IV systems.