The Cs accumulation peaks recorded in lake sediments resulted from human nuclear activities have been served as important time markers and widely applied in modern sediment dating. In the lake sediment profiles collected in the Yunnan area, except for the common appeared main Cs accumulation peaks of 1963, whether there were sub-peaks of Cs corresponding to the year 1986 and/or 1975, the reasons for the formation of those so-called sub-peaks as well as whether they can be time markers were still controversial. In this study, Lake Qinghai in Tengchong, a natural volcanic lake located in western Yunnan, was selected as the major research object. By modeling the vertical distributions of Cs in sediment cores based on the reconstructed regional atmospheric deposition data of Cs, combined with the results of the Pb_ex age data and the distribution of plutonium isotopes, as well as the previous researches on other lakes in the region, the possible corresponded ages of the main accumulation peaks and sub-peaks of Cs in sediment profiles and their formation reasons as well as the influencing factors were comprehensively discussed, and the corresponded time markers of Cs in the region were further identified. The results indicated that the Cs peaks of 1963 and 1986 in sediment profiles of Lake Qinghai in Tengchong were respectively corresponded to the deposition of the global fallout from the nuclear weapon tests in 1963 and the regional fallout from the 1986 Chernobyl Nuclear Accident (CNA), with the contributions of 77%-80% and 51%-62%, respectively, showing the clear time-mark characteristics. While the Cs peak of 1975 in the sediment profiles was predominantly contributed by the catchment-derived Cs with the proportion of 80%-88%, implying that this peak could not serve as a time marker corresponding to the regional fallout from the Chinese nuclear tests in 1975. This study suggested that in the Yunnan Plateau region, the maximum peak of Cs provides a reliable time marker in 1963, and the sub-peak contributed from the 1986 CNA can also be identified as the time marker of the year 1986 for the modern sediment dating in the region.