An interleukin 15 (IL-15) protein complex is provided. The IL-15 protein complex includes soluble fusion proteins (I) and (II), wherein the fusion protein (I) is an IL-15 polypeptide or a functional fragment thereof, and the soluble fusion protein (II) is an IL-15Rα polypeptide or a functional fragment thereof. The soluble fusion protein (I) has at least one amino acid residue mutated to a cysteine (Cys) residue, which pairs with a corresponding mutated Cys residue on the soluble fusion protein (II), or vice versa, to form one or more disulfide bonds. The IL-15 protein complex can be used for tumor therapy.