According to the relief maps, air-screened, remote sensing photos, continuous observation and survey of 29 months, three measurements of the lake depth, as well as data of the lake elevation in May 26, 2003, this paper presents a sketch map of the level variation of Bankog Co in the past 50 years. It shows that the lake level has a rise trend in general from 1959 to September, 2003. However, it underwent a procedure of descent, ascent, and slight descent, namely, from 1963 to 1973 or late, Bankog Co continued to contract and the lake level had a 0. 25 m-full, from 1973 or late to late 2001, the lake level generally rose with an extent of 1. 75 m. Subsequently, the level slightly descended. It is difficult to explain the variation of Bankog Co's water level with its air temperature and small drainage area. The reasons are mainly ascribed to the water level variation of Silin Co,to which Bankog Co connects. With continuous rise of air temperature from 1973, the water amount of melted ice from the Mts. Tanggula (source of the Za'gya Zangbo River) and the Mts. Xianzajiegang (source of the Bairo Zangbo and Xianza Zangbo Rivers) modern glacier, the main water source for Silin Co, has increased. Bankog Co has hydraulic relation with Silin Co. , which leads to the denivellation of Bankog Co.