Glaciers on the southeastern Tibetan Plateau, known as the “Asian Water Tower”, directly affect the utilization of water resources in downstream regions. The currently available instrumental climatic dataset covers only a short period, and the resolution of proxy-based climate reconstructions is low, and these limitations restrict the accurate exploration of glacial changes on the southeastern Tibetan Plateau within the context of long-term climate change. Here, annual-resolution tree ring samples were collected from the oldest pioneer trees (Hippophae rhamnoides subsp. yunnanensis), from the bottom of a glacial valley on the southeastern Tibetan Plateau, and used to reconstruct the glacial changes based on the relationship between the ages of the trees and their distances from the glacier terminal. Results indicate that Zepu glacier has been retreating since 1778 CE, with a total retreat distance of 2757 m and an average retreat rate of 11 m yr−1. The rate of retreat has varied over time, with a maximum of 40 m yr−1 between the 1950s CE and 1970s CE, and glacial retreat has been modulated mainly by temperature variability over interdecadal to centennial timescales. This study provides a new way to reconstruct the history of glacial retreat.