As a complex human-natural system, Water has political and security dimensions in addition to its economic and environmental aspects. Nowadays, the issue of the dispute over common water resources is one of the most important environmental challenges in the world, which can endanger the security and stability of countries. For this reason, water is mentioned as a decisive resource in the relations of countries, and since the solution of disputes arising from common water resources is not the responsibility of countries alone, more attention should be paid to water diplomacy and hydro politics. In this research, the authors have used the case study of Iran and Afghanistan (about the Hirmand River) to identify the role of water diplomacy. The question of the current research is that whether water diplomacy can accelerate the process of promoting friendly relations between countries or not. The hypothesis proposed in response to the research question is that water diplomacy can help to improve relations at all levels as a network of cooperation, and also within the framework of the theory of functionalism, water diplomacy can accelerate the peace-making process and instead of Being the source of conflict is the source of peace and strengthening relations. Also, this article examines the reason for the failure of water diplomacy between Iran and Afghanistan based on the "Access Theory". The method used in this article is descriptive-adaptive and it was done in a library methods and articles, books and websites were used.