The relationship between lake-level changes and lake water salinities has been confirmed as a covariance in closed-basin lakes. According to this principle, lake-level fluctuation history over time can be reconstructed using quantitatively calculation of paleo-salinities in boron concentration of lacustrine mudstone deposition. In this article, boron contents in lacustrine sediments as a salinity index, examples from the Tongtianhe section in the Tuotuohe area of northern Tibetan plateau, vertical lake-level curves inferred from paleo-salinities calculation data was reconstructed during late Oligocene-early Miocene. At the same time, applicability and limitation of this method were also discussed. The results showed that the salinity of paleo-lake water was from brackish to saline water during the upper Yaxicuo Formation deposition. It indicated that late Oligocene was an arid climatic period and lake level was a long low water stand in this area. Conversely, paleo-lake water salinity obviously dropped and reflected water level rise in lower Wudaoliang Fromation. It indicated that Tibetan Plateau climate evolved to enter a humid stage in early Miocene.