Enhancing the flexibility of district energy systems (DES) helps reduce peak load and improve grid stability under high renewable energy penetration. However, existing methods often evaluate flexibility using baseline-free indicators, which lack accuracy in quantifying the benefits of demand response strategies. To address this problem, a two-stage optimal design framework that integrates baseline-required flexibility index is proposed. In the first stage, the operating strategies under baseline scenario and demand response scenario are determined. Then, the energy flexibility is calculated as the deviation between the energy demand under baseline scenario and demand response scenario. In the second stage, a multi-objective optimization approach is employed to minimize economic cost and maximize energy flexibility. The Entropy Weight – Technique for Order Preference by Similarity to Ideal Solution (EW-TOPSIS) method is applied to determine the optimal design solution. Results show that, compared with the conventional single-objective optimization-based design scheme, the proposed method can increase flexibility by 12%. The proposed framework provides a guideline to promote the energy flexibility of DES in the planning and design phases.