Due to the different 240Pu/239Pu atom ratios from different sources of Pu in the environment, Pu isotopes have been widely used for source identification of radionuclides in sediments. In this work, using sector-field ICP-MS, we investigated Pu inventory and its isotopic composition in a lacustrine sediment core collected in Chenghai Lake, SW China. The 240Pu/239Pu atom ratios in this sediment core ranged from 0.166 to 0.271 with a mean of 0.195±0.021, which was slightly higher than that of global fallout. The 239+240Pu/137Cs activity ratios ranged from 0.0155 to 0.0411, with a mean of 0.0215, and the 239+240Pu inventory was 35.4 MBq/km2; both 239+240Pu/137Cs activity ratio and Pu inventory were close to those values of global fallout at 20–30 °N. Three peaks were observed for both 137Cs and 239+240Pu activities in the examined sediment core; they most probably indicated the maximum deposition of global fallout between 1963 and 1964, the fallout from a series of Chinese nuclear tests during the 1970s, and the deposition of resuspended Pu-bearing particles from the Chernobyl accident. Therefore, the vertical profile of Pu isotopes should provide useful time markers for rapid dating of recent sediments.