针对海运空箱调运成本高的行业痛点，本文创新性地将船公司合作机制与免费用箱期策略相结合，综合考虑调箱、存箱、租箱以及交换用箱四种空箱获取渠道，构建了以空箱调运总成本最小化为目标的非线性规划模型。通过设计基于 贪心策略的近似动态规划算法，解决了传统方法难以处理的多阶段决策与非线性耦合难题，给出不同情境下的免费用箱期长度最优决策。研究发现，通过合理设置免费用箱期可降低租箱成本，优化船公司合作下的用箱成本。灵敏度分析表明：当空箱供大于需时，船公司空箱管理总成本将达到最优；而发往内陆箱量每增加33%或运输时间延长1天，需同步缩短免费用箱期1-2天以实现成本最优。该协同决策框架有助于提升船公司空箱资源周转效率，并为船公司免费用箱期长度的制定提供依据。

In response to the industry pain point of high cost of ocean freight empty container repositioning, an innovative combination of shipping company cooperation mechanism and free detention time strategy is proposed. Taking into account four channels for obtaining empty containers, namely container repositioning, storage, rental, and exchange, a nonlinear programming model is constructed with the goal of minimizing the total cost of empty container repositioning. By designing an approximate dynamic programming algorithm based on  greedy strategy, the multi-period decision-making and nonlinear coupling problems that traditional methods are difficult to handle have been solved, and the optimal decision for the length of free container period in different scenarios has been provided. Research has found that setting a reasonable free container detention period can reduce container rental costs and optimize container usage costs under cooperation with shipping companies. Sensitivity analysis shows that when the supply of empty containers exceeds the demand, the total cost of empty container management for shipping companies will reach its optimal level. For every 33% increase in inland container volume or 1 day extension in transportation time, the free detention period needs to be shortened by 1-2 days to achieve cost optimization. This collaborative decision-making framework helps improve the efficiency of empty container resource turnover in shipping companies and provides a basis for determining the length of free detention period for shipping companies.