Abstract: The implementation of natural coastline retention targets is essential for advancing marine ecological civilization in China. However, protecting natural coastlines inevitably limits coastal development and increases opportunity costs. How to achieve these retention targets while minimizing the associated economic burden has become a key issue in integrated coastal zone management. This study quantifies the conservation costs of mainland coastlines from an opportunity-cost perspective and applies a cost-effectiveness analysis framework to develop a minimum-cost conservation model under a fixed retention target. The model results are compared with existing conservation solutions to evaluate differences in economic efficiency and to provide policy recommendations. Results show that the opportunity cost–based minimum-cost model can accurately determine the optimal conservation proportion and spatial configuration for each region under the overall retention constraint, achieving a rational allocation of shoreline resources. Taking Fujian Province’s 2025 retention target of 37.7% natural shoreline retention rate as a case study, the model-estimated annual total cost is approximately 20%–24% lower than that of the current plan, indicating a significant economic efficiency advantage. The proposed cost optimization approach effectively reconciles ecological protection and economic development, provides quantitative and operational decision support for coastal natural shoreline conservation, and introduces methodological innovation in cost accounting and protection solution optimization, offering scientific reference for coastal zone management in Fujian and other coastal regions.