In order to study the water supplement amount and critical period of river ecological restoration, this paper takes the Guanting Shanxia Section of Yongding River as an example, and uses environmental water requirement and ecological water requirement methods to calculate the ecological water requirement of river ecological restoration at different stages. The results show that the Yongding River ecology is seriously degraded and the implementation of ecological water supplement is imminent after the hydrological variations. The current ecological water supplement of 1.7×108 m3 has a certain positive effect, but it still cannot meet the normal water demand. The lowest ecological water supplement of the next three typical years (75%, 90% and 95%) are 4.88×108, 6.11×108 and 6.37×108 m3, respectively. We recommend that the critical period of ecological supplement is from March to June. This research provides a certain theoretical basis for the ecological restoration of Yongding River.