The global optimal capacity configuration optimization design of diesel-electric hybrid multiple unit (EMU) power systems is of great significance for reducing the whole life cycle economic costs of EMUs. To address this issue, this paper analyzes the composition and working principles of diesel-electric hybrid power systems. Based on this analysis, a convex optimization-based collaborative optimization method for capacity configuration and energy management strategy of diesel-electric hybrid power systems is proposed. The power system is modeled using convex optimization techniques with equation relaxation to solve for the optimal combination of capacity configuration and energy management strategy that minimizes EMU energy consumption. To verify the correctness of the proposed collaborative optimization method, traction calculations and energy consumption analysis are conducted based on actual route conditions of an export-model diesel-electric hybrid EMU. Results show that the proposed collaborative optimization method achieves a whole life cycle operating cost of 29.13 million yuan for the EMU, representing a 1.6% reduction compared to conventional solutions, thus validating the correctness and effectiveness of the proposed method.