With the continuous growth of highway quantity in China, the post-maintenance of highways has become a focus of highway management, attracting widespread attention in the industry. Selecting an appropriate maintenance timing is of significant importance for highway maintenance. Based on the TOPSIS theory and the deterioration law of asphalt pavement performance indicators, this study takes cost, effectiveness, energy consumption, and carbon emissions as research indicators. Combined with the entropy weighting method and the analytic hierarchy process, the study establishes a decision model for determining the optimal preventive maintenance timing for asphalt pavement. This model determines the best timing for preventive maintenance of asphalt pavement through the values of relative closeness. Finally, by analyzing a practical maintenance case of a highway in Jiangsu Province, it is found that when the Pavement Condition Index (PCI) equals 90, it is the optimal timing for preventive maintenance of asphalt pavement, which verifies the rationality of the decision model and provides a scientific reference for selecting maintenance timing.