Qinghai Lake is the largest inland brackish lake in China.Hydrological data show that in the past 100 years or so,especially in the recent 30 years and more the lake level has been declining.This phenomenon has been payed great attention,to by the scientists.In the paper, authors try to elucidate the reasons of rapid decline of lake level on the basis of water balance principle.The results are as follows:l.The fluctuation of the lake level is closely relative to the supply and loss of the lake water,with a multi-relative coefficient of 0.95.2.The multi-annual average shortage of the lake water is 450 million m³,and the accumulative shortage of lake water has the same tendency as the fluctuation of the lake level.3.The artificial consumption water occupies only about one percent of total loss.Therefore,the main reason for the falling of the lake level is no other than the natural factor.In addition, with the relative analysis the grey system and the addition model Tthe future of the lake level is predicted. Finally,the limit of the lake level decline is discussed.