Yongding River, as the mother river of Beijing city, has been dried up from Sanjia Dian since 1980s. The naked riverbed and the desertification of its both sides play a main role of sandy weather in spring of Beijing. The Quaternary period underground water of the west Beijing has been drained off, because of the overload of the underground water plus with no supply pouring into. The ecological system of Yongding River has severely been devastated. If we want Yongding River to be recovered, we should clarify the quantity of the most active and the most important water. This paper calculated the instream ecological flow in three monitored sections of Yongding River Guanting Reservoir with wetted perimeter method. The three sections are Guanting Reservoir (under dam), Yanchi and Lugou Bridge. Respectively, the flows in the instream of each section are 3.7 m³ s^-1 (the normal flow year, P=50%, 1978) accounting for 20.7% of the average annual flow, 4.1 m³ s^-1(the normal flow year, P=50%, 1981) accounting for 20.1% of the average annual flow and 1.3 m³ s^-1 (the normal flow year, P=50% ,1978) accounting for 22.1% of the average annual flow. If the supplies are according to the calculated flows, Yongding River will return back from it. Still according to the Tennant method, the Yongding River will maintain a health situation.