The ¹³⁷Cs dating technique has been applied widely in Lake Qinghai, one of the highlighted areas of environmental studies in China. The investigated vertical profiles indicate the ¹³⁷Cs content in the upmost surface sediment (the upper 1 cm layer of the surface) is about 60 Bq/kg (2/3 of the peak value), which is much higher than the supposed content caused by 137 Cs from atmosphere in recent years. In order to investigate the source of 137 Cs isotope and understand the process of the sediments forming, this paper focused on comparative studies of ¹³⁷Cs inventory both in background samples and in the sediments of Lake Qinghai. Comparative studies of ¹³⁷Cs content in sediments on the surface and in sediments at the confluence of contributing streams of Lake Qinghai were also carried out. The results indicate that atmosphere precipitation during the nuclear explosion is the major source of ¹³⁷Cs in the sediments of Lake Qinghai, causing 4/5 content of the total ¹³⁷Cs in the sediments of Lake Qinghai, with just 1/5 content from source of dust sands and contributing streams indirectly. ¹³⁷Cs in the upmost sediments mainly comes from bottom sediments. In such case, when the sediment core was used to study the environment of the Lake Qinghai, precautions should be taken when seeking for high resolution.