Abstract: The Mekong River is Asia’s most important transnational water, and the Tonle Sap Lake is the largest lake connected to the Mekong River. The river-lake relationship between them is complicated, receiving worldwide attention. Using long-series hydrological data from seven river-lake control stations, we analyzed the following topics: ① relationship between the water level and flow rate; ② characteristics, conditions and causes of river flowing backwards into the lake, and being replenished by the water from the lake; ③ backwater effects of water flowing to the river mainstream and water flowing out from the lake. The results show that: ① the lake has characteristics that are similar to lacustrine facies in flood season and dry season. ② water exchange between the river and the lake is extremely frequent; water flowing backwards to the lake occurs in flood season every year with duration of 122 days, and the total volume of 37.7 billion square meters; the average duration to the river is 244 days with total volume of 71.1 billion square meters. Differences in water level between the river and the lake, and the water level of the Tonle Sap River are the two major hydrological conditions for the water exchange, and it is the result of the law of equilibrium in potential energy and kinetic energy between rivers and lakes. ③ there is a strong interaction between the water discharged from the river and the lake. For every 1000 m3/s increases for the river during flood season, the water level would rise by 0.14 to 0.21 meters; at the end of flood season from mid-to-late September to the end of October, for every 1000 m3/s increase in the lake, water level of the river would rise by 0.01 to 0.43 meters. This study can provide scientific basis for the comprehensive management of Mekong Delta and Tonle Sap Lake in Cambodia.