Climate change and human activities affected water supply and water management, and brought a lot of water issues, such as drought, flood, water shortage, deterioration of water environment, etc. In order to fundamentally increase water resources allocation ability, improve ecological environment quality, and enhance the capacity to withstand flood and drought disasters, in China, a new national strategic plan on the constructing river system network was put forward. We have called this plan as IRSN (Interconnected River System Network) plan. To grasp the strategic thinking of IRSN, this paper, based on the basic concept of river, lake, and water system, discussed the hydrological characteristics of rivers and lakes, and analyzed the role of rivers and lakes in the river system network, and pointed out that the river is the main component and lake is the "connector ", "converter" and "storage device" of constructing river system network, and reservoir can be seen as an artificial lake. How to keep the river water mobility and continuity and play the role of lake water storage capacity and ecological benefits as wetland, are the keys to IRSN plan. This paper presents four basic water cycle issues that should be concerned in the implementation process of IRSN plan. First is about water balance. As the structure and characteristics of the natural water system were changed by engineering measures (such as water diversion project, construction of dams, reservoirs, etc. ), the original water balance must be broken. We need to re-examine the new relationship of water balance of the changed river water system. Second is energy balance. The connection of rivers and lakes should be done according to the conversion law of water, the potential energy and kinetic energy, and as far as possible using water bodies own energy to maintain water mobility. Third is about water resources renewability. Just improve river mobility to enhance water resources renewability in river water network. This is the most basic goal of IRSN plan. Fourth is about water cycle scale. As for different water cycle scales, the water connectivity rules are different. So, IRSN plan has the characteristics of multi-temporal and multi-spatial scales.As IRSN plan is an important new plan on water management in China, the related theoretical researches have lagged behind the practices. The purpose of this study is to promote the establishment of theoretical system on IRSN plan. The results of this paper will lay the theoretical basis of hydrology for implementation of the IRSN strategic plan.