Underground logistics system (ULS) can effectively mitigate the negative effects of proliferating city logistics by integrating the management of urban freight flows through a fully automated and networked underground infrastructure. Although the considerable benefits of ULS have been well recognized at a macro level, the lack of quantitative research on ULS benefits makes it difficult to accurately measure the investment and return on ULS implementation. This paper utilized the service cost replacement method (SRCM) to construct a systematic evaluation index system for ULS external benefits, and developed a monetization calculation framework considering the attributes of ULS quasi-public infrastructure. Empirical data from the ULS scheme in Beijing was used to prove the reliability of the constructed ULS external benefits evaluation index system and the rationality of the benefit monetization calculation method. The results quantified the composition of the significant external benefits created by ULS, with the logistics benefits accounting for the largest proportion. The benefits evaluation framework of ULS constructed in this paper provides decision-makers with concrete knowledge of ULS benefits and multi-channel investment and financing strategies, as well as a method of monetizing the cost-benefit of quasi-public infrastructure.