[Objective] To address the issue that traditional methods in the full life-cycle sustainability assessment of highway expansion projects overlook the dynamic interrelationships among economic, social, environmental, and technical indicators, this study establishes an evaluation framework that integrates System Dynamics (SD) with fuzzy multi-criteria decision-making. This framework aims to provide methodological support for the planning, design, and alternative selection of highway expansion projects. [Methods] Based on the theory of sustainable development in road engineering, a four-dimensional evaluation indicator system was established covering economic, social, environmental, and route technical operation dimensions; a System Dynamics model was used to simulate the dynamic evolution of core indicators over the 25-year full life cycle of the expansion project; uncertainty in the evaluation was addressed using triangular fuzzy numbers and the Analytic Hierarchy Process-Technique for Order Preference by Similarity to an Ideal Solution (AHP-TOPSIS) to complete the quantitative comparison of route alternatives. [Results] Using the G6002 Guiyang Ring Expressway expansion project as a case study, system dynamics simulations show that by 2047, the route’s average daily traffic volume will increase by 28.90% compared to 2023, and the GDP multiplier effect will rise by 76.50%; however, carbon emissions will increase by 42.51%, and environmental pressure will continue to rise. Fuzzy AHP-TOPSIS decision analysis indicates that the combined route scheme—integrating local upgrades and expansions with new construction—is the most optimal overall, with a comprehensive proximity index of 0.76. This scheme achieves a balance in terms of investment, traffic functionality enhancement, and social impact, and demonstrates strong robustness as verified by Monte Carlo simulation. [Conclusion] The coupled evaluation method effectively captures the dynamic evolution of full-lifecycle indicators for route projects and addresses the uncertainty in expert judgments, providing a quantitative decision-making tool tailored to engineering practice for the sustainability assessment of expressway expansion projects.