Ecosystem services flow plays an important role in the generation, delivery, transformation and maintenance of ecosystem services. It can establish spatial connections between the supply and demands of ecosystem services. The quantification of ecosystem services flow is conductive to assess ecosystem services accurately, and has great significance on the conservation and management of ecosystems, and effective development of ecological compensation. This paper analyzed the theoretical basis of ecosystem services flow, summarized definitions of ecosystem services flow that proposed by different scholars to analyze their emphases on quantifying services flow, and then classifying ecosystem services flow. Key factors that might influence the quantification of ecosystem services flow were discussed to analyze the interactions among supply, fluxion and demands of ecosystem services flow. The specific process of quantification of ecosystem services flow was proposed, and each step was introduced in detail.