C11b/D8m labyrinth structure can be formed for MoSi2-Mo5Si3 alloys, considered as the promising candidate for ultra-high temperature structural material. For further modifyng the mechanical properties, we recently propose strategy adding multiple elements to the alloy, including element with well-solute to parent phases, while another with segregation to interface. This relies on that the former can modify lattice misfit, and the latter contributes to structural refinement. Therefore, quantitative understanding of segregation behavior and its mechanism becomes essentially important, which has not been clarified for this system. Herein, based on the first-principles calculation with cluster expansion, we quantitatively estimate the segregation behavior for multiple transition elements: e.g., we find that addition of Ta can effectively enhance Ni segregation to the interface, which is interpreted as competition between specific site preference of Ta and resultant changes in Ni segregation. Changes in segregation behavior for other additive elements is also discussed.