Researches are conducted over the problem whether the area out of Tianchi catchment with the altitude higher than Tianchi’s water level can infiltrate as groundwater supplying Tianchi Lake with the approach of water balance method. By analyzing the water balance in 8 basins, conclusions are drawn that there are much unbalance in water around Tianchi basin; Years of average water output is larger than the precipitation; Exotic water recharge 23×10⁸m³of water to the research area every year. Research shows that groundwater supplying Tianchi cannot come from infiltration of areas out of Tianchi catchment with the altitude higher than the water level. For there are no land higher than Tianchi water level in 1 300 km around, it can be inferred that Tianchi receive remote groundwater supplement. Considering recharge area having large leakage and its isotope feature, it can be speculated that groundwater recharging Tianchi is from leakage of Tibetan rivers. There is a special deep cycle of groundwater to complete the supply, runoff and drainage processes.