Tonle Sap Lake is the largest river-connected lake of Mekong River. The water exchange between Tonle Sap Lake and Mekong River has always been a major focus in the international community. Long series of hydrological data of four river-lake controlling stations were employed to analyze the time and magnitude variation characteristics of the water exchange between Mekong River and Tonle Sap Lake. Results show that the water exchange between Mekong River and Tonle Sap Lake is highly intensive. Tonle Sap Lake has obvious peak clipping and dry replenishing effect on the runoff of Mekong River. During flood season (May to September), backward flow from Mekong River to Tonle Sap Lake lasts 122 days on average every year, with a total volume of 37.7 billion m³, accounting for 14.4% of the incoming flow from upper Mekong River in the same period; in particular, the backwater volume from July to September occupies 88.6% of the total annual backwater volume. The average flood peak of backwater is 8 402 m³/s, accounting for 20% of the inflow from Mekong River in the same period. After the flood season, from October to next April, the stored flood of Tonle Sap Lake replenishes to Mekong River, lasting for 244 days on average every year, with a total volume of 71.1 billion m³, which is 1.96 times of the backwater volume from Mekong River and accounts for 29.9% of flood volume of lower Mekong River. Particularly, the replenishing flow from October to next January occupies 83.1% of the total supplementing volume. The interannual variations of duration, volume, and peak discharge of backflow as well as duration, volume, and peak discharge of replenishing flow are stable. The amplitudes reach 76 days, 28.3 billion m³, 6 095 m³/s, and 76 days, 47.4 billion m³ and 4 677 m³/s respectively, with the variation coefficient at 0.07-0.24. The volume of backwater and supplementing water is not only related to the volume of inflow from Mekong River and Tonle Sap Lake, but also affected by the difference of water level between river and lake. The research findings offer scientific evidence for the governance of the Mekong Delta and Tonle Sap Lake Area.