The present invention relates to a pharmaceutical formulation with enhanced stability of a fusion protein dimer comprising a modified IL-2 protein and a CD80 protein. The fusion protein dimer comprising an IL-2 protein and a CD80 protein can not only activate immune cells owing to IL-2, but also effectively regulate Treg cells owing to CD80. When the formulation according to the present invention is applied to a fusion protein dimer comprising an IL-2 protein and a CD80 protein, the stability of the fusion protein dimer is significantly increased, and it can be used as a liquid formulation. Accordingly, the commercial applicability of the fusion protein dimer can be increased.