Abstract: There was a devastating flood in Peking in 6th year of Jiaqing’s Reign in Qing Dynasty caused by the burst of Yongding River. The flood resulted in a tremendous impact，more than ninety districts around Peking were affected. Rooms were damaged, crops were decreased，many people had to leave their hometown. There are many reasons for this disaster，such as the global climate change in 17th century，the special geographical situation of Yongding River. The soil erosion caused by improper reclamation and deforestation accelerated the occurrence of the disaster. Reflecting the ecological and historical reasons of the flood can help us to research the extreme condition of global climate in history and to protect and develop forests properly.