The valuation of ecosystem services (ESs) is helpful for effective ES management and scientific payment for ecosystem services (PES). However, there are many limitations to traditional ES valuation approaches, including the theoretical challenges and the practical difficulties. These limitations cause many difficulties in the practice of ES management and PES. To circumvent these limitations, we proposed a new indicator for the valuation ES, which we termed the ES optional capacity value (OCV). The OCV was based on two axioms: (1) human life would end if we lose any essential ES for human survival (such as oxygen or freshwater); and (2) ecosystems cannot provide any benefits to people without human activities. OCV indicates the optional capacity of supporting the total value produced by a human's economic and social activities (TVPH) provided by the total volume of an ES that was indispensable to human survival. OCV was described by the product of TVPH and the selective degree of freedom. The selective degree of freedom was evaluated by the average uncertainty of selecting the ES consumption from the total ES volume. Then, we demonstrated the new insights into OCV to assist in the understanding and quantification of ES, based on a simple case study that valued the OCV of water provision in Zhujiang River Basin (Pearl River Basin), China. The results showed that OCV could directly reflect the relationship among the total volume of an ES, the consumption volume of an ES, and the TVPH. OCV provided a new insight for understanding and measuring the value of ES. In particular, the OCV provided a clear insight into understanding and measuring the value of ES subsidies between regions. It has potential to form a significant framework for valuing ES as a complementary approach to traditional ES valuation.