With the advancement of large-scale new energy bases and HVDC technology, the remote consumption of new energy via DC transmission has emerged as a prominent research focus. Coordinating and optimizing the capacity configuration of multi-energy combined transmission systems is crucial for enhancing both the economic efficiency and environmental sustainability of these systems. Consequently, this paper proposes a capacity allocation method for multi-energy complementary DC transmission systems. First, a comprehensive model of the multi-energy complementary DC transmission system is established. Second, the K-means clustering method is employed to categorize loads into four representative scenarios, upon which the planned power for HVDC is determined. Finally, a two-stage robust optimization model is formulated, transforming the original problem into a mixed-integer linear programming problem using the C&CG method and KKT conditions, and the Gurobi solver is utilized to optimize the capacity configuration of multi-energy systems. The feasibility and robustness of the proposed capacity allocation method are validated through a numerical example.