According to the measured water level and water input data of the Bosten Lake from 1956to 2008, this paper analyzed the relationship between the water input and the change of water level in Bosten Lake, calculated the design water input in flood season of Bosten Lake by using correlative method. It pointed that main influence factors of changing water level of Bosten Lake is the water flowing into the lake in flood season, not the flood of short duration event. On the basis of calculation, it got the conclusion that in Bosten Lake, the flood level one in 100years under the maximum water capacity is 1048.6m,that of one in 50years is 1048.47m.The result can provide some reference value for water allocation ,operation and management of delivery project of east, west pumping station at Bosten Lake.